Check your induction system (airbox, vacuum lines, etc...) to be sure, but it sounds like you're just experiencing the Volvo M-cam power curve. Torque peaks around 3k RPM, which is what gives you that 'fast' feeling shoving you in the back. HP peaks up around 5400 RPM, but with the torque falling off, you perceive the car to be slower (and it IS accelerating more slowly.) There just isn't much power to be had up there with the M-cam.

The VX cam is supposed to cure this by maintaining the torque curve at a nearly flat level all the way out to 5500 RPM or so, keeping that 'push' going through more of the upper powerband. This makes the engine more flexible and well-rounded.