How important is a good rack?

Racks are a clear case of "horses-for-courses". Some manufacturers e.g. Grand Prix Audio, will insist on knowing what the various loads on the rack will be so that they can customise it by tuning it.
Praiseworthy though this approach may be it only deals with synergy at a simplistic level (i.e. no real-world AB testing with operational equipment)

Turntable support synergy is so unpredictable that the simplest mechanical features can radically alter the sound. I doubt that there are any guarantees that an esoteric table costing megabucks would even top a group table in a 5 or 6 way face-off against bargain priced opposition.

How does an equipment rack remove distortion?

In the last 15 years I saw nearly everything and to make a long story short, when it is solid made, it is ok. Anything more is depending on your wallet. Most of the expensive supports are very overpriced and the advantage in mainly in the belief of the customer....the best investment is in better electronics. Btw. I met Audiophiles who own very expensive racks AND they place their units on a additional shelf PLUS cones PLUS another shelf (sorry, replace the word shelf with isolation device) and on top of that triple they have the unit. Even a triple for SUT's... I bet my car, when the space would be higher they would use 5 devices below a unit.
And, now the ultimate question:
Did it change the "sound" from awful to heavenly?
Hm, all I can say, not for me. But, it is a nice addition and it looks definitely very serious :-)

LOL! (Did Symposium choose the name before ISIS chose theirs? ;^)

There are even worse names in the audio industry though, Schiit for example?
For their sake you always hoped it wasn't... :)

"I married Isis on the 5th day of May". Not a darned thing wrong with that, so why is this an unfortunate name for a product? I know, I know, but really?

If turntable isolation is your priority, and you have a spare coupla-three grand, the Minus K isolation platform looks like the one to have.