Pete, isn't maybe 300 rpm at 22 mph closer? Not sure what prop you're running and what influence that has, but my 1.23:1 gets 43 true with the 13x13 ss 4-blade, and the stock OJ Nibral 13x13 3-blade was about that. IIRC, the SX and X22 shipped with a similar prop, but with the different ratio.
I'm probably 300 rpm short of redline, so you do make 6K, right? Still, it doesn't seem like 300 rpm at WOT would equal 5 mph. I know you're pulling at least 300 moer than me at wakeboard speed, though (to add to whatever down-pitch an ACME would add).
Cross fingers, first weekend out will be next weekend (Havasu). Boat's been sitting since October :-( Tried to go this past week but it was 77 and WINDY. IIRC, yesterday was 107 and clear.