I also have a 750 Sport.
Although boat construction/design is not my thing (I should probably just stick to the massage
), I've been running my 750 Sport for almost a year now and have also been recently trialling some more commercial spec Ribcrafts.
The Ribcraft boys will, I suspect, better explain this, but it appears to me that the hull design of the 750 Sport has not been compromised to afford higher speeds. I have found it to be a fantastic hull which has superbly handled in all states of sea and gives a noticeably cushioned and dry ride.
I would imagine that boat weight is a factor. I would expect the 750 Sport to be a lighter than, say a 7.8, given that she has a sleeker design in general and more "leisure"-type fittings.
I've had 52kts so far out of my 750 Sport - and that's with a Suzuki 250 with 23" s/s prop on the back - with 6ppl & full load and not WOT. Would expect to be able to improve on this. If I recall rightly, a Ribcraft 7.8 with a Suzuki 300 would be expected to make ~50-52kts.
Like RJH, I also trialled several other RIBs last year before choosing the 750. Similarly, I found the 750 Sport to be the most sure whilst still being comfortable.
I'm also about in The Solent most weekends - give me a shout if you'd like a spin to put your mind at rest!