Try a different set of props. See if they have the same rpm problem.

One drive is chain driven, the other is gear driven (to change rotation). The gear drive box is less efficient than the chain drive and will extract more power but I wouldn't think it would 15%. Do you know which side the gear drive box is on?

I believe the ZF 280's are gear only. Basically one is in reverse and one in forward. Typical three shaft transmission with an input, a lay, and output shaft. The side that's down on RPM is the one that has more gear contact to rotate same as the motor. I can't imagine it's 15% though.

They are the 480HP 950TQ rated motors. I just had the the first 250 hour service done on them. We checked the valve lash and injectors, and a few other things. I'm really leaning towards the props, it makes the most sense. My only experience with surface drives is with RC boats. I do know that a very small change in prop makes a huge difference. Granted, that probably not the best comparison.

My Arnesons don't have a gear box. They are connected straight to the trans output. My trans are 1.118:1 ratio, basically direct drive. My motors sit incredibly low and the transmission takes care of the drop. The output flange is also at a 7° down angle.