I started printing 15mm vehicles for AK-47 and used the appropriate files for it. I then went onto printing 10mm vehicles and got the same great results.
Scaling up 3-d files is not the answer – the detail doesn’t improve when, say, up-scaling a 1:100 Sherman to 1:56. Best to use files specifically for the scale you are trying to print.
I’ve never bought a vehicle in the 9 months I’ve had my Makerbot Replicator 2, and now restrict my purchases to figures. I’m sure there will be the day when the quality of these will improve, but currently and with a few exceptions, 3-d printed figures are not good especially 28mm ones.