Like madhatter stated, it depends on how your car is tuned. If you're just setup for a 93 octane tune, then you can just put the 100 in and run more boost. Some people (myself included) have a special "race gas" tune where I throw in some timing in the top end (to keep making power) I would guess right now you could run 18 safely(on 93), you could probably run a few lbs more with 100 octane.