biz-buz:
Good luck. I think you'll find that you need to do a lot of rejected takeoffs, hover in and out of ground effect, and Category A fly aways to prove to the authorities that you have the extra capability.
The issue is that the Flight Manual doesn't give you performance in a meaningful way - by that I mean power required to hover vs. height AGl for a variety of temperatures and Pressure Altitudes. You would then be able to compare this to the (new) power available from the engines to see what improvement might be possible.
You also have make sure the engines you are using a set to 'minimum specification' engines, and not new, out of the box, 10% better than spec power.
A very interesting task - lot of detailed performance work needed.