Hey Pete, thanks for your thoughts. Very helpful.
Obviously the specific brand / design / rubber compound used is important, but if we took that out of the equation for a moment (assume we are talking about the same tire in all configurations), would you say that:
1: tire width and aspect ratio is MORE important than wheel weight when it comes to road course performance?
and Contact Patch. There has to be enough suspension and enough sidewall flex to keep the tire on the track surface. I would trade a couple of pounds for a bigger contact patch and increased control. Now is we are talking slicks the 18s... 
2: the upside of a drop in wheel weight from smaller diameter rims is LESS important than the downside of a taller sidewall, based on what you have seen (again, assuming a road course scenario not a drag strip scenario)?
Yes sir.
It really helps to hear experience talkin'... thanks man.
Pete
PS I just checked the weights:
Forgestar F14s 20x10.5 = 28 lbs (thanks to Nine Ball)
Michelin Super Sport 295/30/20/XL = 29 lbs
TOTAL = 57 lbs per wheel
SAVINGS vs OEM = 4 lbs on each front wheel, 7 lbs on each rear = 22 lbs savings total. In the best 18 inch scenario above I had a total of 32 lbs savings I think. Almost doesn't seem worth it to go for the 18 now that I calculate it out.
