[PrematureApex]

I mean, the 360+ bhp "N55" is a bad example. That isn't an "N55" in anything other than name alone. Different block, crank, pistons, etc.

But anyway, to the point OP, is that OEMs design to a factor of safety, some larger than others. BMW, in particular, builds stronger engines.

Supercharged engines (both turbo and other types of "super" chargers) make it relatively easy/cheap to add power, as they have a built-in mechanism to push more air.

Combine that with the fact that BMW builds very stout engines, and the aftermarket has a big window to offer products which increase power over stock, while still not running the risk of developing a bad reputation for adversely affecting reliability.

It is literally, very, very hard to blow up an N54 or an N55. BMW built them right. So tuners can use that to offer products which significantly increase output, without much risk.