I know this is not about just speed, but since we're kinda on the topic... speed is a topic that's been debated here before, and I always remain incredulous. If someone claims better than 41ish, MAYBE 42ish, from a stock 20+ ft tritoon of any make with a stock 150 of any make, I would have to be on the boat with my own calibrated GPS or witness a calibrated radar reading to believe it. Period. Sorry I've just been around boats too long, and owned too many, to believe "fish stories". And I've been around long enough to see lots of claims fall flat when push comes to shove.
Thinking "well I have a 100 that does 30, so it only makes mathematical sense that a 150 will do 45" is inherently flawed. There are discussions abut this physics of this on the internet that can be found thru Google.
In any event, so he said 39.4 MPH top end @ 4500 RPM. I didn't hear 48. But wait - 4500RPM w/ 2 people on the boat? Again, something is dearly wrong. That Verado WOT range is 5800-6400. If he were running WOT @ 4500 RPM that motor would be so severely dogging he would *KNOW IT* if he's been around boats for any length of time - an experienced boater can feel a dogging engine like a sprained ankle. This would suggest, to me, either the tach is off (happens, happened to me last boat) or the boat is overpropped by 6-8"! The boat would be all but unusable to a seasoned boater. His expectation of 43-44 is "pie in the sky". Read the Merc test, this is what you might expect, ideal conditions, light load and good, tuned-in prop.
The claims we see here of 45MPH tritoons with 150s stock setups I find... highly optimistic. Not to insult anyone, but as the others have said, you should expect upper 30s to low 40s. By low 40s I mean if you hit 41, based on the Merc test of your proposed setup, consider yourself lucky. You better have nothing at all in the boat. And skip breakfast.
I run up along other tritoon/150s on the lakes where boating lanes allow for safe crossing WOT of several boats abreast. Some fall behind me, some run even, some pass, but NEVER EVER has one blown by me, or even passed me impressively. It seems always the Yamahas that run the fastest. And it's always the straked boats that slip past me, few MPH faster (strakes carry weight better too I think, another reason I plan to do it). I also see people claim - and I've said this before - 115s running 36, 37. Me feelings are the same. Maybe a stripped ultralight. There are twice as many TT/115 setups than TT/150s by me. Same drill, anyone who claims they can do so with a stock setup, I only ask that they prove it. I have NEVER EVER seen a TT/115 keep up with me WOT. They catch up to me a minute or two later in the no-wake.
Thinking something in the equation short of extreme modification (removing seats etc) will gain you, OR ANYONE, 6-10 MPH over what others are telling you here, I have to say, humbly, it's wishful thinking. I'm never too stubborn to be proven wrong, but I'm never too gullible to take overly optimistic claims at face value. I've been around boats since age 7 or so, owning my own for 25 years or so.
In any case if you get the 150 you will have a much more versatile boat, but if you expect even mid 40s you will be sorely disappointed. With all sincerity, good luck with the swap out. I hope you do it - and get the numbers you want from the boat.