Nictrix said:
A quick look at what PBondar posted shows that the final drive ratio is also different between the 18i, 20i and the 28i.
It is also different between the 35i and the 35is so that's another difference between them and not just a map.
In fact a bit of digging shows, assuming BMW's literature is correct:
1) 28i has different fd ratio in US vs Europe 3.08(M) 3.73(A) vs 3.08 for both M and A
2) In US both 35i and 35is have fd ratio of 2.56
So BMW spends more time fiddling fd ratio than most other things..I assume this is do to with some aspects of fuel consumption figure plus typical sweet spot for cruising etc.
With a 7 or 8 speed box it can't be particularly about pure acceleration unless dropping the final drive ratio helps overcome wheel spin issues off launch..
I worked in product planning in hi tech and why the spec of a product came out as it did was subject to a variety of issues: dealer feedback, price justification, known buyer behaviour, perceived benefits etc etc..
So just be grateful that you can remap a 18i/20i/28i all to around 280bhp safely...at some point after that you get into increasing probability of it going bang, it seems that 320bhp and 310ft lb seems to be the limit when combined with aggressive/sustained driving....imho
) but it was not very linear and drive able compared to a more "tuned" map with a linear power curve. From my experience the two are hard to combine. Peak power and lots of wheel spin or linear power and more traction. 
