As Arno, James and Bill point out, there could be several sources but your suspicion does seem likely, notwithstanding new mounts. You can easily check if it is a bad engine mount, though. First, stand on the engine and wiggle around with your full weight. If the engine doesn't remain firmly planted you'll easily spot the failed mount(s). (Mine was really wobbly and it was the starboard rear mount!) You can also rev up the engine and look for one side or the other to lift up, signaling a failed mount. Do it in forward and reverse to identify the bad mount.
However, it being only at high RPM and with having just installed new mounts, that seems it is something else like the suggestions to check the coupling. On my boat years after the failed mount and on two other SN's I've looked at it was loose bolts on the Vetus coupling, and that was, indeed, more noticeable at higher rpm, although there was a rattle on deacceleration. Put a wrench on all the bolts and check if they are tight - do the same on the engine mounts while you're at it.
One other procedure I've done after popping my engine up to the cockpit to more easily do various maintenance on it is to put it back in place and just loosely tighten the mounting bolts. Then start it up, run it at low rpm in forward and reverse to let it find it's "sweet spot" and then torque up the bolts. You can do that, of course, without uncoupling it or lifting it out of the engine room. It might just let it "settle in" to where it should be.
Keep us posted - would love to hear the final diagnosis.
Craig - SN68 Sangaris, Tropic Isle Harbor, FL