My '18 came with 20x9.5 rims all the way around with 275/35-20 tires. I don't believe Roush did anything electronically to compensate for the larger-than-stock tire diameter on the front. One interesting thing is my 19x11 Apex EC-7 ET52 rims with 25mm spacers actually have slightly more strut clearance than the Roush rims. If you used the different offset rims that Apex makes for the 19x11 (ET52 rear, ET26 front) along with good camber plates, you could still get away with 305s all the way around, just not with the ability to rotate front to back. You could always mount/dismount to accomplish the rotation, though. I've had several 300+ mile trips with my 305s (RT660, -2.7 camber, 0 toe) and it's actually worked quite well. I did load up the 4 tires in the car for a trip from Kansas to Michigan, though. The 19x10 rims would be a simpler solution, but you'll always have smaller tires than the "serious people". I did a fair amount of information gathering before I got rims and decided that because I autocross, the taller rim/tire setup was necessary to keep me out of 3rd gear for autocrossing (top speed is still over 70 in 2nd with the 3.73 gears and the '18+ transmission).
As some others have said, I think tires will do more than suspension. My before/after going from 275 Continental Sports to 305 RT660s netted about a 5 second improvement (give or take - not done scientifically) on a 60 second autocross course. It can be debated how much of it was tire size and how much of it was going from a 340TW to 200TW, but whatever it is, the results are definitely much improved. I upgraded my front brake air deflectors to a version of the Vorshlag units. They helped keep temperatures in check on the road course for me (although I'm definitely not as hard on brakes as some people are). I'll probably switch to G-loc pads and retain the stock rotor blanks. For me, the few extra hundredths or tenths isn't worth the cost of going to 2-piece rotors at this point. I do HPDE for fun, and my autocrossing will improve way more with driver mods than with slightly lighter brakes.
As some others have said, I think tires will do more than suspension. My before/after going from 275 Continental Sports to 305 RT660s netted about a 5 second improvement (give or take - not done scientifically) on a 60 second autocross course. It can be debated how much of it was tire size and how much of it was going from a 340TW to 200TW, but whatever it is, the results are definitely much improved. I upgraded my front brake air deflectors to a version of the Vorshlag units. They helped keep temperatures in check on the road course for me (although I'm definitely not as hard on brakes as some people are). I'll probably switch to G-loc pads and retain the stock rotor blanks. For me, the few extra hundredths or tenths isn't worth the cost of going to 2-piece rotors at this point. I do HPDE for fun, and my autocrossing will improve way more with driver mods than with slightly lighter brakes.
