Chap who went to my local site stripped his piston in less then two days of use running an 11.1v in a UAR, might have been other damage as well.
You need more then a mosfet to make an AEG long term reliable running a higher voltage lipo.
Always thought 11.1v is a bit excessive anyway, 7.4v is perfectly fine. Check out componentshop for a good selection of batteries.