I just test drove a 2014 3i MT 4-door, and I don't see what you're all moaning about. Seriously, the 2.0 has plenty of beans, even with the excellent A/C going full blast, at least as quick as our 2007 3s 2.3 MT hatch. Don't judge based on a bunch of numbers, go flog one for yourself.
We also drove a 2015 6 MT, and the 2.5 in it felt weak, way out of proportion to the ~400 lb. weight difference. It felt like a turbo that was slow to spool up, in fact my wife declared it was one zoom short of a zoom-zoom. With AT you'd probably not notice the 2.5's lack of response, but with MT it was a disappointing slug. We lost interest long before we got back to the dealer. (It also had numbish steering, and wasn't as quiet as the 3.)
I suspect that's why you can't get a 2.5 MT in the 3: it would suck. No one buys a 6 with MT anyway, so that particular match-up is of purely academic interest. It was a miracle we found one to test drive, and I will not be surprised if it's still on the lot a year from now.
The 2.0 is a much better engine IMHO, snappier and ready to pull RIGHT NOW.
milliHelen: the quantity of beauty required to launch one ship.