A lot of guys run the 20hp Tohatsu 4-strokes on their LT25's and they certainly perform well enough. Most are getting right around 30mph out of them. Personally, I prefer a 25hp 2-stroke, which when properly propped, tuned and tweaked can approach 35mph. Of course, you're limited to used with a 2-stroke, and they'll suck up 2-3 times the fuel per trip. As for the 4-stroke weight, I wouldn't worry about it too much. Lot's of LT25 owners put much heavier 40, 50 or even 60hp motors on them and other than a slight draft increase, they perform just fine.