Someone on the 1 series site is claiming they can remap the 120d to 230bhp (which I dont actually believe) god knows what they could put the 125d up to. Some have said the problem may be that the gearbox wouldn't be good for the increase in torque as it's 450nm standard.
In terms of the actual MPG figures. Speaking to owners, the worst I've seen is 48mpg and that was from a guy that only really drives around town. Most seem to be getting mid 50s which is miles better than the old 123d.