I dunno, in pretty much every application I've seen dyno'd, turbo cars seem to "retain" the most power vs N/A cars, experiencing less loss of power through the drivetrain. Then again, I do realize that the cars are have very similar engines and likely also have similar, if not the same, other drivetrain components which would be contributing to the loss of power to heat. I was just going by the fact that most turbo cars I see tend to have stock dynos that are higher than the "15% rule". For example I see lots of stock STIs and even 135i's dyno around 270, which would be a ~10% loss from the claimed stock HP for both. On the contrary, a 425 hp SRT8 stock dynos between 350 and 370 from what I remember and can wrangle up on google, and the G8 GT from Pontiac dynos around 290-300 for the 360 hp V8 engine. Those drivetrain losses are definitely more.
I don't know if it has anything to do specifically with it being N/A, and I do agree with roo97ss that every car is different and every dyno is different and OP's car could be making 220 HP or 250 HP, who knows because we don't know the car or the dyno, but I'm just making some optimistic comments based on my own experience
Roo is correct, though. You don't really know if the dyno reads high or low, and you don't know if your car is higher or lower output than average. Every car is different, so the important thing is to get that baseline. But, personally I think you either got a good car, or all 128i's are a little better than rated, just based on what I've seen for drivetrain loss in other cars compared to these cars. So that's still awesome!