Higher pressures, within approved limits, improve:
a) wear resistance, b) mpg, c) hydroplane onset speed (as in higher), d) braking performance and e) handling (stiffer side walls).
Braking performace can be seen at:
http://www.nhtsa.dot.gov/cars/rules/...FEA/TPMS3.html
Scroll down to tables near the end of the study.
I would also think that higher pressures improve ability to go in snow since the footprint is smaller, and probably degrade traction on ice (same reason, smaller contact patch. Every skinny tired vehicle I've had went thru snow better than my vehicle(s) with low profile, high aspect ratio performance tires.
Hydroplane onset speed (rule of thumb) is 9*SQRT Tire Pessure. A 21% increase of tire pressure should increase hydroplane onset speed by 10%. Going from 35 psi (53
mph onset) to 42 psi should raise onset speed to 58
mph! Again, rule of thumb. The proportional change holds, but tire specifics will result in unkown variance.