I wouldn't be surprised they are the same back box but you get the extra pipe that replaces the centre silencers on the 135i. not sure if it's only the 135i that has the extra set of silencers??
The 135i exhaust is marginally more expensive.
I know what you are saying about a custom exhaust for that price is good but remember it's only custom made because they haven't done one before
The BMW Performance one was also custom made at one point