+1 for all who said the German gunsight rocks. I love this thing, and I've gotten very used to sizing targets at 800, 1000, 1200 and beyond. The real secret is being able to estimate distance between a shot, and where the shot was intended and making corrections. The other HUGE tip I'll give is use your mouse to aim your tank main gun. You can get a thousand times more accurate through leafs and over hills than trying to muscle the joystick around.
As for wirbles... there's a lot of luck, and a lot of lead. The tip I have is if you think you're leading enough, go a bit farther. In an ostwind, lead even more. Those are a crapshoot for me. Some days I'm on, some I'm off, and normally it has to do with the contact coming in.