[kanucks]

TL;DR: M2 is better if money isnt a concern or a major decision factor. But if you are price sensitive and want to have the better bang for buck, then mod a m235i, but at the same time risk losing your warranty and potentially resale value.

Comparing apples to apples, M2 will be 10-15% better (stock versus stock m235i). So the price will obviously be at least 15% more. You can mod both versions, so moot point. No point in comparing a modded m235i to an unmodded M2, unless $$$ is a factor. In which case, you're not really comparing cars but how much money you're willing to spend in return for performance.

The OPs comparison could be flipped around and we could compare a modded 228xi with an unmodded m235i, and similarly the modded 228i would come close to or maybe even beat a stock m235i in performance given the same amount of $$$ spent. But again, that's comparing a modded car to a stock car.

I think the OP is trying to justify a comparison based on performance/$$$ spent, which is still a valid form of comparison, if we are to presume that the limiting factor in some people's decision is available funds to dedicate to this hobby.