I'm not an e90 owner, but i drive an i6 e46 330ci. I've been on my cat-back supersprint for coming to a year now, and i think though the 2 engines are different, i think the principle behind should be the same.
The main difference after installation is without a doubt, a noticeable loss in the low end revs. A free or semi flow aftermarket exhaust system helps to reduce the exhaust output resistance and as a result, mid-high revs will be much more shiok, but on low revs, it's sluggish. Add a mid-high cam plus open pod air intake in my setup, i cannot imagine driving without at least a sprint booster to counter the low rev retardation. With the Velocity available now, it should be even less of a problem.
My other concern was also altering the original exhaust note, since i really liked the stock bassy sound of a 3L i6. Was afraid that after changing to after-market exhaust, it may alter the sound. Turned out to be an unnecessary concern, because it sounded close to stock, but with a lot more depth, and the scream after 4000rpm never fails to up a slow day for me.
Final tuning of ECU will also help overcome the low end loss.
In summary, no regrets for the change. Should work out for the e90s i6 just the same. Do post some reviews if you do decide to go for the upgrade.