My philosophy is this: Tyres are No. 1 - the car is almost secondary.
When the r35 was released; the first question i asked was: what semi-comps can fit under the car?
Answer: 295/35/18's RE55's (thank god)
As soon as i had that answer, i purchased the car.
There were no 20" semi's available at the time, and over a year later the only 20" semi available (R888) is significantly slower than the RE55 still.
I would not have bought the car if more affordable/faster rubber was not available. Further, I won the 2009 NSWSS championship (by a single point) using that tyre, albeit last round on slicks. I simply would not have won on the 20's
FWIW, the RE55's are roughtly HALF the cost of the 20" OEM tyre and 2/3 the cost of the Toyo 888. The 2nd hand slicks i use are a fraction of the RE55 cost again believe it or not (GT series throw aways). So it may be counter intuitive to some but in this case the 18" semi and 18" slick options are far cheaper than 20's and much much faster (several seconds per lap) and just more apprioriate really. The tyres allow you to get so much more out of the car. And i have saved many thousands by using 18's - In this market, cheaper is often faster, more appropriate and more robust (despite the cost argument people throw at say rotas for example, you need to experiment somwhat). Anyway if you want to go quick, my belief is that you select the tyre FIRST and find a capable car that can accommodate the tyre second. I spent years with commodores etc with serious power and some setup that just couldnt fit a decent tyre under the car; it just didnt work. The R35 i reckon will probably fit a 330mm slick under there without too much additional work, the times we are going to see in the coming two years will knock your socks off if we can make that happen.
If you want a nice road car that can run good lap times for the occassional track day - the 20's are probably good enough. If you do a lot of track days (go through lots of rubber) you need a better option. That better option happens to be cheaper vs OEM (as it often is).