Do stands make a difference for equipment?

Agear, at the end of day most stands should be seen as a design element. When you put some Sorbothane devices between Speaker and Stand (no matter which one) you have a very good working solution without spending big bucks.
When you want a real vibration transfer, the speakers should be screwed into the stand, no one does it, so you will find all kind of window dressing :-)

" Total cost, about $500. How much better can a expensive stand improve on this?
..."

Excellent question. I can only relate actual experiences and not anecdotal experiences.

When my hardware gear (amp and Cdp) went from $4K to $24K the stand and isolation tweaks eventually went from $850 to almost $5K with no other stand tweaks. The speaker cables and ICs also went from $2K to $6K also.

Did I Gulp and sweat it? - U betcha. But did it make a difference? Again U betcha... and a very noticeable improvement difference - Go figger!

Acoustic room treatments with Echo Busters became the integral matched steps. Room nodes, first order reflections, bass damping and reverberation times et al all needed attention, along with tinkering with the speaker - to - listener positions .

Can I directly parse out with near accuracy just much of the overall improvement was attributed just to the stand?

Hmmmm ... not exactly, other than to say it was not subtle. Again I was the ultimate skeptic before.

Oh, did I mention that for any of Alan's facebook friends, each Q-stand (what I use now, and will be buying more of soon) are only $350.00 each? Better hurry though, when Alan's new and only B&M store being built in Nashville comes online in the next few months, they will be a few thousand dollars each, according to Alan.

When you want a real vibration transfer, the speakers should be screwed into the stand

Not necessarily, or at least not as long as you believe in the Star Sound scientific perspective - simply bolting speaker to stand will not effectively accomplish "real vibration transfer".

Syntax,
Having tried Sorbothane padding vs. small pieces of blu-tac, I must disagree. You may prefer the Sorbothane to other interfaces between speaker and stand, but I didn't, and they certainly sound different.