I've used GRUB to dual boot before but I find that in general I always boot into Ubuntu unless I'm going to play a game that's not Linux supported.
Two main questions here
Can I set up the boot loader to always boot Linux unless I "opt-out"? Either by setting a flag on disc when I restart, or having to press an f key at a specific time.
Would I see a noticible difference in gameplay performance if I run windows off of a VM?