I've been there and done that. The truth hurts, and there is a very good chance that the GPS is correct. 52 mph sounds a little bit slow, but a 21 foot Omega is a pretty heavy boat as well. If you aren't absolutely sure about the condition of your jet drive, you may be giving away a lot of speed. Also, if you let the engine spin a little bit higher, you'll probably get to where the horsepower peak of the engine is. That means a smaller impeller and a blueprinted drive. Of course, the hull condition and straightness plays a big role also.
Just for a quick comparison, my CP Gullwing has run a best of 82.6 mph on GPS with a roughly 475 horse 454 at 5200 rpm.
SteveM.