1) I have the same question as DanielOR. You get a fair bit of kit to begin with, and are free to sell whatever you can't/don't use. For me, that includes Shotguns, Smoke Grenades, IR goggles and Shiva cannons. Also since the budget is so tight the first month, try not to get anything extravagant until the first of April.
2) It might be possible to edit the .ufo files, but it wouldn't be easy. Also, there's no guarantee it wouldn't corrupt your saves. As Daniel-san said, first look for optimization. It might even help to reboot before playing...
3) It's currently not possible. I get better performance through giving each soldier a weapon that matches what they are good at.