My first autocross I was in a stock 1988 944 Turbo S. I got 2nd....from last. The only person with slower times than mine was a girl in a 914 that kept getting lost in the course. Even with my slow times I was told I won specifically "Because my smile was so big you could see me smiling even though I was wearing a full face helmet."
Took a couple of years of both setup changes and learning to drive in anger before I got fast enough to start setting FTD. Upgrades included a second set of forged club sports with Goodyear R1s.
I don't know about the availability of 16 in Rcomps, but the 16in Club Sport wheels that came on the 944 Turbo S are very very light and look more correct on a 944...Not the Design 90, but the Forged Club Sport Wheels.
I did go to 18in Carrera lightweights for more tire choices.
Going up in rim size from 15 to 16 and 16 to 17 slows acceleration by .1 seconds 0-60 per inch of increase. It also increases cornering G's by .2 G. With the same brand of tires. However, this is not true when going from 17 to 18 because only better tires are available for 18s. The acceleration time does not increase, they stay the same as the 17s. With 18s the cornering G's increase by the same .2 G. This was according to testing result done by one of the car magazine, don't remember which one though.