Where is the significant point of diminishing returns on hi-end turntable?

I think the point of diminishing returns moves along a spectrum, depending on the rest of your system, how "dialed in" it is (which is something that takes time after living with various components and getting the most out of them) and the nature of your record collection.
In the '80s, I had a Well-Tempered Turntable which, at the time, gave a huge amount of performance for the money and it was fine with a first class cartridge over a system with limited bandwidth and dynamics (Quad electrostats). When I moved to a bigger, wider bandwidth system, I changed to a more conventional, substantial table- a Kuzma Reference with Triplanar arm and a selection of good cartridges. But the revelation, for me, was upgrading that Kuzma for the XL (a high mass table) and the same manufacturer's air bearing arm (using the same cartridges), which made the following differences:
less sense of a turntable playing- no 'halo' around the sound or other artifacts that tell you a reproducing mechanism is in play;
deeper, more natural bass- sounded more like instruments than just low frequencies.
This is most obvious on good sounding records- not the "audio porn" which can sound spectacular for "demo" purposes. Once you start playing music you like rather than stuff that shows off your system, you'll hear a big difference between different records and different pressings of the same record- some will sound far flatter and lifeless in presentation than others.  When you listen to a wide variety of different records, you get a different baseline than just "demo" testing gear on a few impressive sounding records. I think this is where the differences in front end start to be revealed: not differences in overt colorations, but the ability to extract more information from the record without having an analytical sound (the reason, I think, why most vinyl lovers cling to the medium). But, the big variable, all other things being equal (which they never are)* is often the records at that point. 

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*The turntable/arm/cartridge/phono stage combination is probably the hardest to audition in the process of buying, since there are so many variables and so few opportunities to make direct comparisons. 

Given how difficult it is to make turntable/arm/cartridge/phono stage comparisons in a meaningful way--a point I touched on earlier--CT0517's advice above is pretty good- get with some people--civilians, not dealers-- in your area-West Coast?- who have some top tables and spend some time hanging with them. Most audiophiles I know are very generous that way. You can thus avoid the dealer bias -- I'm not down on dealers, generally, but their job is to sell you what they offer-; you also want long time user experience, which can be invaluable. 
I "get" where you are coming from in terms of not wanting something fiddly but still being able to do routine maintenance and adjustments. 
If you are looking for something to buy and keep, and not be worried about the 'next best thing'  I'd do exactly what you are doing: researching, and spending some hands-on time with some of these turntables. 
I have my favorites and preferences but see no need to name check brands here- there are a lot of good tables and arms out there. 
good luck,
(I was a car guy too, and have fond memories of hurtling around the track in a CGT with Hurley Haywood as my coach for a day). 
bill hart

Davey- yes, and the same thing comes up (at a slightly different cost/risk) with certain valuable old pressings. I choose to play them rather than keep them on a shelf for posterity. As far as public roads go, it's the wild west out there! Drive safe. 

Rush- thank you. You spared me some unnecessary typing. 

So, do you think, with the wide range of views about where the point of diminishing returns kicks in, it is fair to say that the answer varies-- depending on a number of factors (system, record collection and budget, among them)?

I’ll throw out one provocative thought regarding the "money issue" which seems to be an undercurrent here-- that obscenely expensive tonearm that a few reviewers, starting with Fremer, have raved about--the SAT? (i think that’s the brand name). Crazy money, right?
But, what if, by using it, you can use far more modest cartridges? Top tier phono cartridges are nutty money today, and are "consumables"- so the cost of several cartridges would offset the extravagant price of the arm.
One last thought--for now. I have a fairly big league table and arm. It isn’t the latest and greatest and that is, in some ways, the point--I haven’t found a need to upgrade, modify or replace it since I bought it circa 2006-7. (Replaced a few belts and the manufacturer supplied one replacement part at no cost since I’ve owned it). In the long run, cheaper than trading up or sideways periodically.

Lewm- how wonderful that you had stewardship of that car. Thanks for the reply. I'm not a "Porsche guy" (though I've owned a few of them and have enormous respect for the marque). The question--not to derail this thread- is whether you'd be willing to put such a valuable car in traffic today. I admire the people who do, but it takes a certain mindset. Good for you! 

lewm- you had a real 550 Spyder? Did you own it a while ago, before these cars reached Van Gogh painting prices? One of my heroes is Ed Niles (not even sure if he is still around). He bought, imported, drove and then sold (to fund further acquisition of) some of the great sports cars of the '50s and '60s --bought them when they were just tired old oddball cars on a professional's income (Ed was a lawyer). 
It is a fun hobby. Nice post. 

Issues over the OP's user name, Porsche and any sizable amount of money spent toward a turntable started with the first response. There has been an undercurrent of skepticism toward higher priced turntables in this thread and the discussion about sports cars seems to have added to it. But, as one of those who posted on both subjects (high end turntables and sporting cars) in this thread, I ask you to look for any hint of snobbery, elitism or "wealth worship" on the part of those who made such contributions. In fact, most of the folks I know who have spent oodles on turntables are not rich people--they just have different priorities. Likewise, many of us who have owned exotic or vintage cars are not necessarily rich (I pointed out the example of Ed Niles, above, who was a work a day lawyer with a keen recognition of the value of obscure sports cars long before they became cherished museum objects).
Perhaps the topics are analogous- both represent embodiments of mechanical engineering and design, sometimes to a very high level. But, don't confuse that with worship of Mammon. That's certainly not where I'm coming from and I doubt some of the others who contributed to both topics here are encouraging an elitist view.  You can enjoy these things without rancor.
bill hart

If you go that route, you might have AF make a couple extra arm mounting boards, finished as you want, so you don’t have to try to match finishes at some future time.