molotov, a good rule of thumb to follow is to use the widest tire on the widest rim allowed within your class without compromising gearing.
simple question parisifal; why do you suggest 15 inch wheels? Ive been trough some posts here and there and i see ryan in a video cornering cones with 17 inch mags. Also , i saw piers14 in ireland who also goes for bigger wheels size. Also, i have been to some time attacks in my region and the difference with my original "sunflower" mags and the 17 inch that i bought after is way bigger in cornering and grip.
A large portion of the difference you felt was because of the shorter, stiffer sidewall of the 17's. The rest was likely because the 17" was just a better tire compared to the 15's.
I'd also bet the 17" tire is wider.
I run 17x8 because it allows me to put the most rubber under the car on the widest rim my class allows. I run a 235/40/17 because it is the best compromise between tire width and it's affect on gearing.
If I had my way, I would run 15 or 16x8" rims...HOWEVER nobody makes a competitive 235 or 245 width 15 or 16" street tire.
another question; Why 35 psi front and 30 psi back? I asked myself the same question in the track and 30psi grips a lot better. 35 psi makes your rubber way too hard.... i gained myself 2 secs less in my lap because of that.
Are you SURE it was tire pressure, or did you just drive better?
Your optimal tire pressures will depend on several factors, the tire, width of the rim it's on, your suspension/alignment setup, the course and the day.
Info on tire pressures:
Tire Pressure Guide - Tech - Sport Compact Car Magazine
Start with chalk, or shoe polish on the shoulders of your tire. Start high on pressure. Optimal pressure will allow the tire to roll over all the way to the shoulder without going beyond.
At some point that may stop working as it did for me when I went into STX. I started using a pyrometer to measure the temperature of the inside, middle and outside of the tires to not only set pressure, but camber as well.