I can assure you that my mafless 75 does significantly more than 30mpg....
The 160 rover 75 map was made by a bunch of guys on the 75 forums. It's designed for different injectors, turbos, exhaust diameters all sorts. And people seem to get mixed opinions on its efficiency.
A mafless remap with all variables programmed typically operates at about 95% power efficiency. That will remain constant as a maf isn't needing fixed etc.
A maf enabled remap will operate at 100% power efficiency, however as the maf degredes that will drop a lot.
Its quite common for many of us to have dual maps on our cars, one with a mafless file and another with a maf file. Again it's been done for nearly a decade in the ni Mg rover scene now pioneered by Mad Tuning and Reidy Remaps.