At 6300revs, you're 100revs over the max.... although peak power from what I remember is between 5800 and 6200revs.
If your usual number of people on board is 5 to 6, and she doesn't rev past 6200 then there's probably little point in changing the prop. However, if normally you run with 2 to 3 people on board, and are hitting 6300revs (can't be far off the limiter?) then the next prop up in size may benefit you and your engine. 50mph is pretty fast for a 5.3metre rib!!
The main thing for me with the stainless prop is that it doesn't dink quite so much as the ali prop. After a good day out, I could come back and you can see a few tiny chips on the ali prop, whereas the stainless doesn't get affected quite so much. In my case, 21" ali to 21" stainless hasn't given me any more speed, so the lack of dinks is about the only benefit to me anyway.
I think the next ali prop up with Suzuki is 23" - if you look up prop calculators here (its been discussed a fair bit before), and bung in the Suzuki specs you should get a rough idea of what you might get out of her.