Theoretically more sidewall allows you to put power down better too. Putting down power isn't a strong suit on the supra. =D Whether it does in reality, I'm not too sure, although I did recall Reg Riemer mention that switching to 17 in the rear helped. Not to mention the 285 17" is taller up front and tends to rub.
You can fit a 285-30-18 in front, and a 315-35-17 in the rear. There are almost no 315-30-18 tires available, so 18" rear is tough, and a 275-40-17 is too tall in the front, so a 285-30-18 is an excellent choice at an inch shorter. That is THE reason. Max footprint in the stock body with proper ride height. Carl Byck
I was one of the 18/17 pioneers years ago. The 285x30x18 in front is superior to the 275x40x17 for two reasons: it's shorter (no rubbing issues) which provides very quick steering response, and it's about an inch wider (more grip). I debated about moving to 18's in the rear, especially given industry trends toward 18s and 19s, but John (CCW wheels owner) and I agreed that the short sidewalls on most 18" tires would be a disaster on a powerful Supra. The 315x35x17 offers more sidewall flex than a comparable 18" - allowing for better straight line "hook up." That said, in an ideal world (i.e. I had flared rear fenders), I'd love to run a 335x35x17, or maybe a 335x18. Note that I run mainly R (race) compound tires...
Also, you may be refering to the Jun supra that has the 18" fronts and 17" rears that is in the infamous drifting vid. I have read in an article on the yellow jun supra that in their case, the 18" were put on to clear their 6 pot Alcon brakes, which the 17" would not have cleared. I am pretty sure they would have had the 17" all round if they could, with the yokohama A032R on all 4 corners as the A032R is a better tire than the A048 they put on the front rims. A048 come in 18", A032R only come in max 17"