With an AUW of 7500kg, we have a payload of 2500kg. Not great when compared to some of the pure helicopters in that range!
Do you mean 'payload' (i.e. fuel is already accounted for) or 'disposable load' (payload plus fuel)? Ballpark figures for 2 or 3 types in the 5-9T range I know of give disposable loads of around 33% of MTOM - which is the figure you come up with for payload. So, if true payload then your figure would seem good but if not it would still seem reasonable given the performance benefits over a true helicopter.
Are you looking to certify in CS 25 and CS 29? One of our problems is that a tiltrotor doesn't fit anywhere, so we spend hours trawling the documents for gotcha's!
What's the plan for the AW609 certification basis - seems daft to reinvent the wheel? I don't think anything exists at present, but I believe the plan is for a kind of blended Part 23,25,27,29 solution giving a new category of "Powered Lift."