How important is a good rack?

Rack suggestions are very subjective.
I don't know of anyone personally that has done a comparison of racks in the same room at the same time. I'm sure that would be interesting and difficult to do.

I like your idea of the wall-mount rack for the turntable, that can be very effective and convenient. As far as your Michael Green rack goes, I use one solely for the flexibility in adjusting the shelves, but I would not put a turntable on it, only power supplies and equipment that is stationary without moving parts (like a music server). For the time being you should tighten the nuts holding the shelves in place, that will make it a lot more sturdy. The MG theory of his racks was to channel all the vibrations from the equipment down the metal rods holding the thing together through points into the floor; I don't necessarily buy that one, and have used various footers or Symposium platforms when I have had components on it.

The main rack I use for my TT is an old Merrill Stable Table, which I filled with lead shot and sand and weighed in at about 350 pounds when full. It is very effective, and cost about $1100 when it was new, which was a lot for a rack back then. I would not spend $6600 for a rack, though, and any reviewer that calls that "affordable" is delusional, sorry. Racks can make a difference, but I do not think you need to go overboard on them. Just my $.02.

I put the turntable and preamp on two seperate shelves on the wall. Amp and Dac on the floor It made everything sound better!

If you can try it, I went to home depot and bought some shelf brackets that are 20 in and good for 1000 lbs, bought a nice finished piece of birch plywood and the results were amazing.

While weight of a rack can matter it is engineering that matters most. Engineering along with quality of build and appearance is what you pay for in an Sra rack. Check the price of any high quality furniture. My wife bought an entry table to our home. One shelf. Small! $1200. Now that is outrageous. I understand all we have different price points. There are many ways to get from point a to point b. None better then SRA.
Oh and to the point of this thread, yes racks do matter.
I'm just saying ✌️

"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.