1,2,3,4 just above your WASD, are good ones to change for things you need fast. Some people don't like to use those, but they work great for something you need quickly and briefly.
I use 4 (& double 4 to hold) for my compass, and 3 for my laser/light accessories. It's awesome with a bright flashlight, as you don't take your hands off your trigger or movement keys and you can just blind the hell out of some fucker as you breach a room.
The numbers are only used for "Command" sub menu, but they serve no use to me as that so I just deleted them for that. But yea wait till after OSUT and find what works for you by just sitting down going through the control settings.
(Also yeah double G for grenade for the love of God. DO IT NOW!!)
As for the latter question, I don't think anything is used to increase performance. If you have a decent PC, I wouldn't worry too much, you should get around 30. If you're getting shit frames, chances are that everyone else is too. That seems to be usually what happens.
Good luck on your OSUT Day 4!