Playcraft holds the record at over 80 with twin 300x's on the back of a 26. I believe the boat is in Havasu at one of their dealers now.
I have a 28' Bennington with the 350 mag and it will run 45 mph. I did not but it for the speed though, I bought it for the bar, BBQ, 20 person capacity, not having to worry about beaching it etc... They are also available now with a 496 HO.