The way I fly it is thus- Go heavy, stay light. I fly the D-11 mostly.
The Jug will burn tons of gas, and she thrives on the thin air, but she doesn't climb all that well.
The heavy ammo load should really be reserved for ground strafing and/or bomber interceptions. Those are the long protracted high speed dives where the extra weight won't hurt, and you're not pulling on the pole non stop.
The point of the Jug is air superiority. Come in with an advantage, force the bad guy to deplete all his energy, pin him down, then whack him over the skull.
You can't do that if you're constantly watching your fuel state. You're not supposed to be turning all that hard (and if you're flying the Jug correctly you really shouldn't be anyway), so you should always be losing energy in minimal states through your zoom climbs.
So, to recap- cruise at around 15-17K. Typically, that's how far your drop tank will take you, so pop it off to clean up, and you'll be cruising at a resepectable 380ish, punch wep and you're creeping up on if not exceeding 400.
From that position of advantage you should be able to attack with impunity.
Once your fuel gets down to 50%, you can get a bit more aggressive. At 25% you can furball with the best of 'em, but keep an eye on your altimeter. You will need 5,000 feet and WEP to clear out of a fight.
In an N, it's all a bit different. Save the WEP for when you need it, take 50% gas, and a drop tank. Same rules apply for the most part, but if you punch wep on the deck you're lookin at a significant speed advantage, and if you dive with it, you can run down a Mustang in short order.