Re: 2006 GX205 5.0l Top Speed/prop
Can't give you much advice on the prop specs. but I've gotten better than that with mine. Obviously depends on your load, but with me and couple kids, I've gotten up to about 48. Don't think I was quite WOT though, so pretty sure I could hit at least 50-52 with it. And you'll want to be sure you're trimmed up as well to get top speed.
There are a lot of discussions regarding prop types on here though. Try searching the forums, or you may want to post in the general boat topics forum. You'll get a lot more responses there on which prop will do what for you.