The sidewall maximum is 35PSI? That is pretty low -- most tires have a sidewall max of 44PSI.
I'm running Nokian tires in the summer, and I have Michelin X-Ice for the winter. You should check out the Nokian eNTYRE, which is a 51PSI sidewall max:
Nokian eNTYRE
Yes, the fuel is shut off when you use the engine to brake by lifting the throttle or downshifting. Your SGII should show 9999mpg when this happens.
If/when my clutch needs replacing, I will try to see about having the whole final drive case replaced with the 3.56:1 unit. My xA has a 4.31:1 final drive. You should check what your final drive is -- it may already be the taller geared unit. The manual shift cars have the short gearing because *if* you get cruise control, then you don't need to downshift. Automatics probably have the taller final drive, but since they are 4-speeds the overall gear ratio is only a little better than the 5-speed manual. Both 4th and 5th on my car are technically overdrive, but it spins at about 3,500RPM at 65MPH.