Mine is a custom built (homemade) 23' triton with lifting strakes on all three toons with a mid 90's Johnson 150 HP. It will run in the upper 30's mph but I've never seen 40. I have a 6" jack plate that's probably set up similar to what's been mentioned and, I installed one of the lower unit extensions from Bay as mentioned to make it a long shaft motor. Then raised it slightly with the jack plate to get the right motor height. If you are as low as you can go and it's still ventilating, then that's about it without an extension.***
You didn't mention if the toons have lifting strakes. If not, you can probably pick up some speed by fixing the ventilation problem (I think it's ventilation rather than cavitation in your case), but I doubt you will see mid to upper 30's without strakes. Based on my experience with building this boat with at least 5 different configurations of motors, toons and strakes, I don't think putting a larger motor on it will help much without strakes. I'm guessing you don't have strakes. A pontoon "hits the wall" at a point if you can't get it on top of the water and more HP just plows harder and is not very productive.
*** I also tried several props (which would make it 7 or 8 configurations) and the one I found the best for my boat is a big ear "off shore" 4 blade prop from Power Tech. It really helped with the ventilation problem I was having but didn't fix it all the way without the Bay lower unit extension. The extension and a 4 blade prop will run you $1,200 - 1,500 if you can install it yourself, and I'm certain you can find a motor height that won't ventilate but without strakes I wouldn't count on mid 30's mph.
As mentioned, please list the rpm's it's running and the WOT rpm's on the engine rating. We can be much more helpful with that info. Normally they post these topics in the prop section but personally I'd rather see it here since it's on a pontoon.
Hope some of this helps,
Mark