Just-In Time, to check if the engines spin to rated rpm's, the boat should be loaded full, since he is not getting the last 300rpm's at 50% he may not be proped correctly, fill the tanks and run it again, if he does not meet the manufacturers specs, then the boat needs new props. Spinning up to speed does not mean max rated, just within Yamaha's specs. If you are on the low side of that range, new props would not be a bad idea but are not essential. Also, what does the bottom look like, is the paint newer and smooth, or old and has cracks and chips, that can also be a cause of some lost rpm's. Also, how were the engines and tabs trimmed? Test the boat with no tabs used for trim, just for a list, and trim the engines up to find the sweet spot. If the engines are trimmed fully down, you will lose some rpm's also. To find your sweet spot, get to cruise speed, and then trim engines up and watch your rpm's and fuel burn, find the best numbers and then punch it to see what she spins to.