Hmm, I'm having a similar problem. When I'm just cruisin down the road at say like 50mph and I step on it to either pass, or to get up a hill the boost guage will go up, but my car won't downshift and speed up. I really have to punch it to get it to go anywhere.
I've only had my car since July. Last month I had it tuned up, with AC Delco plugs, Taylor 8mm wires, K&N drop in, new fuel filter and o2 sensor. I am still not thinking that my car is putting out the power that it should. I haven't yet "serviced" the SC. Is there a specific type of maintenance that should be done to it?