My response to the top speed post was simply information I gained from 4 of the potential 6 mercury engine re-builders in the country. Each and every engine rebuild company I called to fix mine asked how hard I run it. These 240 mercs have ring locating pin problems at high rpm's. Ring moves and tears the cylinder up causing engine failure.
First I've heard of that, but could be true. Sounds like you did your research.
However anyone taking on a new jet boat with a 240 merc should be cautious as to not think it's bullet proof.
NOTHING is bulletproof. That seems especially true in marine equipment. Remember what B.O.A.T. stands for! But I will say that my Mercury has been far more reliable than my Rotaxes. Just yesterday, in fact, I was debugging a fuel system problem on one of my two Rotax engines. It's fixed, but that's another couple of hours I'll never get back that were spent working, rather than playing, at the lake.
I am taking my engine builders advice of keeping it below 5000rpms ( 45mph ) to make sure I have a boat to use all summer.
By happy accident, we run our engine at 4000-4500 RPM most of the time these days for wakeboarding. It does get the daily WOT run, but 95% of its time is spent below the threshold you've specified. Perhaps that's why ours has been so reliable? Not sure, but not complaining!
Side note: I wonder if warmup has anything to do with it. I'm careful to warm up all of our engines, starting with 10-15 seconds at idle, then bump 500 RPM for another ~15-30 seconds, then another 500 RPM, etc. I'm constantly amazed at people who cold start their engines and immediately ask them to accelerate hard, or run WOT, or whatever. That just cannot be good for close tolerance parts - like, for example, rings and pistons.
Hopefully this rebuild will last me longer now that I am much easier on it. 
