We are all from the UK.
I did notice you mentioned earlier about the Rotax engines,I rate them very highly,its the old Lycoming / Continental engines i`m not such a big fan of.Didn`t mean to sound like a snob!
As for the 260 Hp limit,that would not really be an issue,as the subaru engine develops maximum power at 7300 RPM,with a T3 turbocharger. An idea is fit a smaller turbocharger,decrease the boost and limit the revs with an electronic limiter,and tune the engine for efficiency.The reason we want a comparitively mad amount of HP,considering C150s and other 2 seaters only have 100 or so,is the design criteria is speed,and modest range.The engine would help alot with both of these aspects.
As far as we know homebuilts in the UK can only fly within the UK on a "permit to fly",so as it would not be making any foreign trips,we could divert more attention to giving the aircraft a higher cruise speed and due to this reason,the size of the plane would best be described as a tight fit!! Not too dissimilar from the profile used in a glider (reason being the smaller an object is,the faster it moves through the air,and less drag created) ,and a rear mounted pusher prop will be doing the work.
Thankyou for the replies btw.