The 6 liter 400 HP motor is plenty. I wouldn't base your decision on Chris' experience at 5300 feet. That's going to make a huge difference. Also, for the best wake behind the MXZ, avoid putting too much weight in the front. It flattens it out.
We ran the first ever prototype MXZ / LS3 all last season. We ended up with stock ballast, wedge, 120 in lead on each side of the motor, 600 in the floor pressed up against the rear seat, and 500 in lead sitting in the walk through right underneath the windshield. Nothing in the nose. This configuration produced a wake comparable to other boats running much, much more weight. And for this application, the LS3 was way more than enough engine. The 6 liter will get it done without a doubt.
I also rode behind the 350. If you're going to add 2500 extra pounds, this probably isn't the best choice. It will get it done, but you'll have to be patient.