How important is a good rack?

I found the difference to be night and day. I had now idea how bad my first two racks were until I tried a better one. No turntable here, but the difference had nothing to do with rigidity of the frame. The worse one was more rigid. The better one did not improve with more bracing. Again turntable may be very different.

"I'd be very interested to hear accounts of people
doing serious ABs comparing a "merely
competent" rack (say like my homebrew rod and block)
with a SOTA rack, say like a Finite Elemente, which costs
10 times more."

I went from Flexy to ikea lack to Finite elemente. I
guess that is like a/b/c. I wasn't serious, though, but I
did not have to be. The lack and finite are both much
better than the mdf-baced flexy, which was the heaviest
and stiffest. (Stiff in relation to the legs with
least stiff and heaviest shelves).

"What I was wondering about whether investment in
racking yields cost commensurate improvements comparable
to other elements in the chain, once the fairly modest
"threshold" I suggest has been reached. My
speculation is that for many of us, the money might be
more noticeably placed elsewhere."

I disagree because I do not believe its possible overcome
the problems of a bad rack elsewhere. I am also not
putting my stuff on the floor, especially since my stereo
is in my living room.