you would cast your bullets at .309 with a standard mold, the coating would bring it to about .310 usually, maybe more. but it doesn't matter, because your going to be running your cast bullets through a sizer anyways. and yes, it is recommended to shoot .001 over your bore size to reduce leading by means of hot gases passing around the bullets base. also, it will be more accurate if it's big enough to fully squeeze into your rifling ensuring a straight flying bullet as it leaves the barrel.
but along with your mold, you will also need to buy a .309 sizing die. or .310 if your lug your bore and find it to be bigger than .308, although doubtful in a quality gun like a garand as long as it's not shot out
powder-coating GREATLY reduces any lead fouling. mine holds up to 1800fps w/o gas-check so far. I am sure I could go faster too. check out cast.boolits forums to see what people are shooting with PC, they are really pushing the limits