tire pressure
Running at 15 pounds going up to 30 will it effect the 60ft
Any change in tire pressure or tire diameter will affect '60 and RT's. The amount may not always be noticable because of all the different variables, but the difference will be there.
(i.e.: YOUR reactions, precise location of the car on the line (shallow/deep), side-to-side location of the car relative to any crown in the track and/or rubber build-up, etc.)
Personally, I think 15 lbs is asking for trouble. I worry when I have to go down to 25-26lbs because a track has an extra tight rollout.
As for slicks, larger slicks tend to need less air pressure. But I've seen 9" slicks run at anywhere fron 12 lbs to around 30! Personally, I found 21 lbs to work best in my Hoosier 9" lightweight radial slicks.
