My cousin had a similar setup on his VKing several years back.
He got 98mph running a 14.5" x 30 pitch cleaver.
Hippie actually created a thread about his boat.
http://hydrostreamforums.com/viewtopic.php?f=5&t=472
I threw up a video on YouTube of me in it with him driving (obviously) back in 08. Its crappy video quality and over 7 minutes long.
I was trying to get closeups of the tach and speedo but it was a bumpy ride. I eventually gave up and got some over-the-windshield shots.
We got around 85mph. Medium chop on the lake in spots.
But that Merc screams at 6500+.
https://www.youtube.com/watch?v=9qphMYoolxY&t
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He says that your 84 sounds about right for your 26 chopper. Says you'd probably get 100+ with a 30-32 chopper.
And you shouldnt need a jackplate with a 15" mid.
Or are you talking about setback instead of height?