You are correct in your feeling of the car, it is a little more tricky to get used to the power that is added by the turbo charged engine. It takes more practice, but once you get the hang of it, it can be more fun and or very controlled. The tires
make a big difference and I have gone to some sticky larger 255/35 18 Hankook tires
and they are very sticky and I have a lot more grip and the car is easy to control in the corners.
The general consensus is that the 135i pushes in the corners, it has understeer, that is easier to handle than over-steer, so its not too bad. That being said, the car does have a short wheel base and it can come around on you :-)
I love the extra power with a tune and its a blast to drive. I put it back to stock last week and it is very boring to me in stock form, but you put that tune on it and its a blast to drive, I am loving it so far !
Played with some RB turbos last week too, now I know my car will be getting some more mods soon !