I think HPK and injectors along with a revised calibration would be the easiest route, and quite achievable in my opinion. The stock RS injectors are at the upper end of their duty cycle in the current spec, so 550cc's are an obvious choice.
The MR400+ concept specs were very appealing to say the least, however if that path came to market it would no longer be a case of "send us your ECU", but rather "send us your car", for the internals to be done, and that would no doubt be a VERY expensive exercise. Of course the reliability would be second to none! But as they say.. you get what you pay for.
So Rico.. when are you heading over to Essex and returning to your PC to report back to us mere mortals?