I once drilled out my tear ducts while the decals were attached. I used the engraving attachment to make a series of small holes as guides, then connected them with the circular cutter. The vinyl was more or less alright but did pull a little.
If I had it to do over again, and may with my new TD, I'd probably slice out the decal with an x-acto along the lines I was going to cut, then make the cuts with only the circular cutter. It really lets in a lot of air, even without the fans.
My favorite mod, one that I've never seen anyone else do (without having first seen mine) is to cut out the mouth (not the frown). I cut out the vertical grooves on the mouth, then back it with mesh. It again airs the helmet out really well and also allows your voice to be better-heard.
I've only done it with an FX helmet, so it's no huge deal. I understand any helmet with the mouth pre-molded could be tricky if you screw it up, but it's a really rewarding mod in my opinion. There's also plenty of room there for a fan.
If anyone's interested I can post a pic.
Phil