I actually do have some experience of running a well tuned Plus 8 albeit with a Rover engine. Mine was an ex race car that I had done up for the road and sprints/hillclimbs. I had all the road gear put back like hood, wipers, etc. Mine was only 265BHP but about 900 kilos (1980lbs). So power to weight was 0.134 (Mogvette is 0.16). However this was more than enough for very inspiring road performance and on track as well. In one sprint I was the fastest covered wheel car beating Porsche, Ferrari' etc. Even with road legal competition tyres this would spin the wheels in third in the dry and every gear in the wet. It needed treating with utmost respect on the road other wise it would bite back hard. It also had full discs all round and properly located and damped back axle. The other thing it had was an integral roll bar and many racers these days use full cages which help brace the chassis as well. I and many others consider this about the ultimate usable power package for a road going Plus 8.
I have also briefly driven a full Class A Plus 8 race car on the road and would not like to do it again. This had 400BHP from a 5.0ltr Rover V8 and suspension/brake set up similar to the Mogvette. It was a complete and utter beast. You could never realistically use full throttle on the road. Basically great for the track but no point on the road.
There has got to be limit where the basic body/chassis cannot reasonably handle more power (torsional stiffness and aerodynamics etc). I think 400BHP on the road is way above that limit.
I would also say that with this amount of change you cannot call it a real Morgan anymore as well.
