I have P1s and they rock for handling!! I have them in 18s and using Khumo 712s. I gladly gave up a few tenths in acceleration to get alot more grip and stability. My 2cents.

 

the change from 16 inch wheels to 18 inch wheels reduces sidewall flex and effective roll from the tire. The increase in width is also beneficial to stability . Physics of less side wall will not allow flex if it is not there. Thats why most tarmac rally car preparations are using larger diameter wheels with lower profile sidewalls. Thats my best assessment of it anyway.