I've been flying the 109G6 with 30mm cannon over the holiday break and I'm impressed with the firepower it can dish out. It'll climb real well with WEP but the turning performance is weakened with all that weight in the nose. Still, 1 hit from the 30mm will knock down a fighter, 2 hits will knock down a medium bomber, 3 will take out a larger bomber. Best tactic is to use it as a BnZ fighter. Climb to 20k and make sure that all of your attacks are made from the top going down on your target, then using autoclimb to get back up because you'll probably be pretty close to compression if you dive more than 3,000 feet or so.
The shells are slow moving and ROF is pretty low, so get in close, give it plenty of lead, and only fire the 30mm - no real need to waste your 13mm MG ammo when the 30mm will knock down a plane with 1 to 3 hits, so save the 13mm for the low-percentage snapshots where you're just trying to puncture a radiator or knock off a flap.