Compound meaning the material that makes up the tire. Hard street tires suck and going from a 255/35 to the max size on a 20x9 for example probably won't make much difference on a supercharged car. I don't know what size you run now.
Are you looking for a street tire or drag radial? If I remember correct I thought you had another thread where you said you can't install drag radials where you live?
With your power levels you really need a 275/35 drag radial if you a staying with a 9" wide wheel. They are much softer than street tires so they will have better traction but they won't last as long.
How many miles do you drive in a year? The nitto nt555R is what I run. As far as drag radials go they will provide a bit more miles of use than other drag radials. They are not the stickiest of drag radials but they will be better than a street tire.
Something to consider at your power level. I think people see around 15k miles on nt555r's depending on how you drive.
A 20" drag radial for our cars with a 9" wheel is going to be a little difficult. 275/40R20 is the only one that comes to mind. That tire will be nearly 29" tall though.
Spacers won't help. Don't go wider than 275 on a 9" wheel. MPSS are not as sticky in a straight line as the same size drag radial, but will last much longer. It will also not handle nearly as well, as drag radials are meant to flex in the sidewall.
anything you put on that 20"x9" wheel isn't gonna give you much of a gain in traction other than a different tire compound like the nitto nt555r, and im pretty sure even that wont give you the amount of traction your looking for.
they make the nt555r in 275/40/20 so get a pair and try em out, just be careful in the rain.. and if your gonna get a whole new set of tires don't put nt555r's on the front. if you want nitto's all around put the nt555's up front. the R is the drag radial.
your absolute best bet for max grip and maintaining the ability to drive on the street would be going with a 20"x11" rim in the rear (if you really wanna stick with 20's) and getting the nitto nto5r 315/35/20.
if you don't want to go with a drag radial and seriously cant get a new set of wider wheels, consider the Michelin pilot sport cup 2's in 265/35/20 or the Dunlop sport maxx race in the same 265/35/20.
good luck bro, try to get some 11" wide wheels.
and yes if you add 1.5" spacers it will help your stability and traction while cornering, but wont affect your straight line grip.