I have an 8.5 Scorp with 275hp and get around 49 kn with a 19 inch prop, the 21 gives me 50 kn+ but seems a little sluggish at cruising speed acceleration. My 8.1 stepped hull Scorpion which was a pretty light boat and ran a 225 Opti was capable of 53 kn so looking at things I guess that you wot top speed is about right. The interesting things is that the 8.5 is so much smoother that in the real world it is a quicker boat over a distance with varying sea conditions but the way it delivers the speed is so relaxed it feels slow. For me the 8.5 is a much better boat than my 8.1 was.
However the easy way for you to hit 50 kn+ is buy one of the new 300 hp engines that are out there!!!! Oh and you could buy me one at the same time if you are feeling generous.