yes, it's typical of basically All crate motor co's to under rate their power levels, as is the case with probably all vehicle mfg's. it's a safety feature for them so that nobody can ever claim theirs didn't make the claimed xxx hp they advertise. i'm not sure how much they under rate, but it's enough so that any variation from one motor to another is covered. let's say a company builds 20 motors. they're not all going to make the same power due to this or that, even with the exact same parts right down to the bolts. if 1 motor makes 300 on the low end, and another makes 315 on the high end, and the other 18 fall somewhere in between said high and low, if they advertise 290hp they are more than likely never going to have a problem with a customer if they should have an engine dyno at their disposal to check the claimed hp level and find out it's lower than they claimed, and now you're posting all over the net that they're f'n thieves, f'n liers, f'n whatever. sorry about the rambling, but does that make sence?
for example, i called up World Products a few years ago asking about a 540 that was said to make xxx hp, and he flat out told me it Will make more than that, but that's how they have to advertise. give GMPP a call and see what they say. they might have the avg. power that they typically see out of them.