I would certainly change it, Minis should never have been given a diff so high, the gearing is all wrong. Its trying to be so economical that it can't accelerate as you would expect, consequently you plant your foot and bang goes the economy...and the acceleration
Like many Mini related mechanicals its personal choice and I would be going 3.44 in an MPi especially as its mildly tuned.
3.44 was standard issue all 1275 Cooper S with options to 4.1
Why are they the most desirable and fun ??
The irony is the Mini emerged out of an oil crisis, and was designed to be economic and went on to be [and still is] a giant killing performance car and then was killed off with the fuel economy / emmissions obsession.