Ive had two working in an experimental dual drive gearbox and it is possible to push them to 24v, but be careful with the heat ive always found them to still get fairly hot with a computer fan on. But ide recommend running them at a safer voltage which is what I am now doing with them for tests. (I wasnt aware that you had them Stu !!)

Also ive ordered some HTI Replacement motors and going to try them out, only a couple of $s more but a slight increase in HP output and also able to take 24v. Only downside is at 12v there is a slight drop in rpm compared to the Mini Ev at 12v. So might be worth a look.

Chris - http://www.featherweights.org/forum