Slicks on a 991 GT3?
 

Has anyone run Pirelli Slicks on a 991 GT3?

I currenty run Hoosiers in 265/35 19 and 325/30 19 on 19x9.5 and 19x12" wheels.

If so, what sizes? And how much faster vs. other tire options?

Which Pirelli slicks? There are many options: H - HARD, M - MEDIUM, S - SOFT, SS - SUPERSOFT.

Which Hoosier are you comparing them to? R80, R100, R7, A7, etc.?

Generally, the soft Pirelli slick will be 3-4s faster per lap on a 2min track over the softer Hoosier based on Laguna Seca data and Pirelli World Challenge cars. Neither type of tire performs at its peak for very long. That time is assuming the driver can get the most out of both types of tires and makes the appropriate suspension changes to maximize each set-up.