Rega spindle too short

It isn't a problem unless you want to use a clamp in which case there's not enough spindle to grab onto. The Regas are challenged that way anyway which probably suits Rega's Roy Gandy just fine. He enjoys scoffing at anything he can manage to write off as superfluous.

You could try making your own mat out of something you could cut out a relief for the label. I wonder how difficult it would be to take the platter to a machinist to turn a relief in it. Bad design choice on the part of the Iron Audio.

"They would have to raise the price..."

yup.

.010 - .020" oughta do it. If forgoing the felt reveals enough of the spindle to use a clamp, you'll want to use a spacer of about 1 1/4" in diameter that fits over the spindle and protrudes above the level of the platter another .010" or so, depending on the record and how much spindle is available. If you can manage it, you'll like the result.

The benfeit of the recess is the same regardless of the platter material. Snug record/platter mating is a very basic no brainer. It is a major factor in just about every aspect of the sound. Consequently, IMHO, clamping is major. I was just at Musical Surroundings listening to the $100k Clearaudio Statement table that uses a ring clamp as well as a center clamp. Even with this MASSIVE assault and its center clamp, the difference was not subtle when the ring clamp was added. Even with the Rega and it's felt mat the improvement was noticable when using the clamp from my Maplenoll as a record weight lending more transparancy, solidity and body.