I would recommend taking the car to a remap company that has a rolling road / dyno so that you can get a before and after printout. My car went to Performance Torque at Wolverhampton but Evolve and Celtic tuning are also popular. If you use the search facility on this forum and type in n20 remap there’s plenty of information available.
I wouldn't, it will cost a lot more for no real benefit IMHO.
Welcome Smudger. A white E89 does look lush (biased I am).
Remaps for the N20 are ten-a-penny. Not heard of a bad one. I used Superchips on my drive for £300 and have absolutely no issues whatsoever.
In contrast I had a s**t remap from Celtic via one of their dealers..it wasn’t run on a dyno so spent two years wondering why the mid range was so crap..
I’d always pay the extra to have a dyno..then you know for sure ref before and after..
You’d think they are all the same but it’s interesting to note the variations…
FWIW later 20i with electronic waste gate tend to make more power on re-maps..