The 328i has the same engine and chip, the only difference is the rear section pipes on the 125i. It is only from the cat back that the 125i is smaller.
If you replace the standard pipes with 75mm tuned pipes from the cat and the rear exhaust back section you will increase the output (cost is around $2,600)
There are several specialist Exhaust shops around that can make the pipes and change the exhaust to mimic the 328i's 75mm system. That will increase the output.
If you want to wait until the 328i M tune chip comes out and then get the full exhaust changed to suit it, you could get 255kw (estimated). Not sure if you really need all that extra horse.