I'm pretty content with my 35i with MPPK Tune, (M performance power kit) and racechip ultimate tune chip. It's pretty fast both off the line and in the upper speeds.
According to BMW a 2015 and up 35i is said to do 0-60mph in 6.1 seconds! MPPK tune shaves 0.5 sec. from that time to make it in just 5.6 seconds for 0-60mph. With my racechip tune stacked, I'm going to be conservative and say at setting D, it shortens the 0-60 mph to 5.1 seconds. Consider this.. BMW is notorious to be very conservative on their performance numbers, where many car magazines have seen faster 0-60 times than stated by BMW. An X5 50i sprints from 0 to 60 in what 4.3 seconds?
In regards the exhaust, I do recommend MPE (m performance exhaust) which is OEM and it does sound much more aggressive than stock.
Here are a few video of mine on YouTube for reference.
Good luck.