GT-40p headed motor

[Shawn]

Sounds like the springs to me also..one rule is to always expect advertised hp numbers to be exaggerated a little bit. But in your case the valves are probably floating quite a bit, one clue is that you can sometimes hear an almost clicking/ticking sound in the higher rpm range if the springs are weak-thats the valve winging open to far and the spring slapping the valvetrain back down. Springs are cheap, anywhere from $50-200 depending on how crazy you get. And dont let anyone tell you they have to take the heads off the car to replace them-just so they can charge more $$$. I used to change springs all the time on one of my high lift cam engines, there are a few tools that easily let you change springs with the heads on, if you do it yourself just be careful to make sure the piston it 'up' in the cylinder your working on-that way you dont have to worry if a valve slides down!
good luck!