Hi there, I have a 2010 Cooper D Convertible. Cant really comment on high mileage as I don't do more than average miles per year and my car only had 4k on it when I got it.
MPG I think is good, you are not going to get the 80mpg or whatever is quoted unless you are driving in a vacuum. When I use mine for work I get about 60-64 mpg, that is over 26 miles and on A Roads doing between a steady 50 - 60 MPH. When I drive back from London I tend t stick the cruise control on at 75mph and I seem to be getting just on 60mpg - all according to the on board computer.
When I first got my MINI I was disappointed with the mileage as I seemed to be constantly filling it up compared with my Corsa but then realised the tank on the MINI is smaller. The MPG has also improved over the time I have had it, now that it has done just over 30k.
As for the lights - if they are not the xenon's then yes they are crap and when I get my next MINI I will try and get one with xenon's. Motorway driving generally is fine, as is city driving, the worse bit for me is when I am driving unlit roads and country lanes, the output just doesn't seem adequate. I too have changed to Osram Nightbreaker, they are marginally better but still not brilliant, saying that though, for me it would nto be a dealbreaker to put me off a MINI.