Do you see alot of "black dust" on the S/C snout?
If you do, then that could mean that there is significant slippage.
Are you running a 5% or 10% pulley?
There are a couple of things you can do about slippage, one, maybe your belt is due for replacement anyway. But you might want to go with the goodyear gatorback belts. I've heard good things about them and I know that they are very quiet.
Also, I have the steel tensioner pullies. They do quiet down the belt noise significantly.
The steel tensioner pulley for the superchager belt is 1/8" larger diameter, which means it is going to put a slight amount more tension on the belt. It's tougher to get the belt on the car though, but I did it with my stock pully. You might have an easier time with the smaller pulley.
Consider a slightly smaller belt too.
A loss of 5psi, at a certain RPM does sound like belt slippage, but it could also be something else.