You'd have to turn at least 5800+ rpm with a 19 pitch prop to see 50mph.
http://www.go-fast.com/Prop_Slip_Calculator.htm
That's not an easy thing to do, cause most 225's are big heavy 3.0L + motors. If you find a Yamaha 2.6L 225 or a excel or 220 special . . . Those will hit 50+ with the v20. I actually think the boat would be slower with a stock 3.0L Merc 225 vs. a 2.5L merc 200. 200hpdi . . . would probably hit 50mph. The extra 5mph takes exponentially more HP to reach once you get above 45 mph.