I spoke with a guy at POD some time last year, he owns a tuned A45 and he walked me through some of the problems, not insurmountable but would need some thought and a company to spend on the development to overcome.
I would imagine Merc have over engineered the car with tuning in mind, just curious to know how much the drive train can cope with or would you have to dial out torque in key areas of the map compensate.
To be honest 500bhp with 4WD in a light compact car such as the 45 is more than enough for a street car of this size, it appears to be a car that handles and the chassis can be tuned to accommodate a power hike, would I have the 45 over something like a Evo10, yes I think I would.