You know while I had the 5% on mine, I gained at least 3 lbs. of boost. I do think the belt slipped some when it got close to a shift point. You might want to try a belt just a bit smaller. I don't know how much gain you would get from a 10%, but you would definately need a smaller belt for that one.
Yea I read somewhere the proper way to increase belt tension is to buy a bigger tensioner pulley but a smaller belt should work too. I heard you lose low end or mid torque with a 10%.
I can't wait to try it out. I haven't driven a SC in 7 years from the last one I had. This one was bought with blown HG's. Now I'm waiting on a couple parts from T-bird88 to finish the full rebuild. I will use the stock pulley for the engine break in just to be safe then switch over to the 5% after about 300 miles.