I know exactly what you are talking about. I don't push my car too often either because I have always felt like the manual gearbox in the Crossfire is clunky which gives an impression of it being fragile. However, someone discussed this before but on a dyno, I believe the Crossfire's power band is somewhere between 4500rpm and 5500rpm. If I am really "gunning" it, I put the pedal all the way down and rev it to around 5300-5500rpm and then quickly shift up which usually brings the rpms down about 1000rpm to 4500rpm. I typically continue this process until I want to slow down. At 5500 rpm the car should be able to shift pretty easily BUT make sure not to over rev as this could obviously cause serious problems.

My other thought about you shifting at 4200rpm or so and getting a slow response is that for some reason the manual gearbox in the Crossfire has serious throttle lag (I think thats what it is called). I have always noticed this. It basically feels like it takes a second or two longer for the car to "pick up" the next gear than it should. Its an annoying feature and I believe many forum members have purchased a SprintBooster which solves this problem. The SprintBooster reduces throttle lag but does not lower 0-60 times or or increase speed but rather improves the feel of the shifts.

As someone previously mentioned though, press the pedal all the way down and I would recommend revving to 5500 rpm and shifting continuously. Let me know how this goes.