Maybe it is in my mind, but for ME I'd rather be heavy vs an average corsair "for example" then light. I find the "zoom" potential is improved. I agree that "heavier" is worse in general and for E retention, but using high energy state and zoom to manipulate the fight has more to it then just pure E retention IMO.
JUGgler
Just to confirm what BnZ said, you want to fight the Jug as light as possible. Higher weight is nothing but a performance penalty, regardless of what you perceive. In all models, regardless of total fuel quantity, I like to be at about half a tank in a fight. Anything more than that and it wants to mush. Luckily, that gas guzzler up front will help you get there in a hurry. That being said, I usually take off with 75% in all models but the D-11, where I take 100%. But, that's accounting for the fuel I use climbing up and cruising to the fight. Learn to get back to base with 1/8th of a tank, and when you're at 1/4 tank or so, all models really start to get nimble, relatively speaking, especially once you've blown through half your ammo or so (because you're carrying several hundred pounds of ammo as well). With all the extra power, I really like fighting Ns and Ms at 1/4 tank--they can really power through a hard turn until they get completely out of E.
I personally set my convergence at 300 on all guns, unless I'm doing some dedicated buff hunting, then I push them out to 400. That being said, the most lethal guy I know in a Jug, Wolfala, likes a shotgun spread on his Jug, starting at 375 and increasing each pair by 25 yards up to 450 on the last pair. So, I'd say do what works best for you.
Remember that 7,000 feet gives you enough altitude to accelerate to 500mph in a dive, which will allow you to escape most everything else that's not already doing 500mph. Never use more than 3 notches of flaps in the beginning. If you can't get the shot with 3 notches, you're probably not going to get it.
The Jug has a ton of rudder authority, and sometimes I'll actually skid inside of a turn to get a shot against something I can't quite out turn. About the only things you can out turn are 190s and P-51s, so don't try that until you get savvy enough.
When doing classic BnZ, make sure your zoom climbs are unloaded. Pull a few G's to get your nose up, relax the stick, and then let Pratt and Whitney do the rest. I recommend never letting your airspeed get lower than 200-250 indicated during the zoom before you level off or get the nose back down to recover speed. The Jug really wallows at low speed, so to preserve some maneuverability at the top of the zoom, spare some airspeed.
Against bombers, use very high angle 6 o'clock attacks. Executed properly, the top gunner positions can't track you as you drop almost vertically into the formation. The caliber .50 works best against the bomber wing roots to start fuel fires instead of pumping rounds into the fuselage.
Last, stay as fast as possible as long as possible the whole time you're up. Always have some airspeed held in reserve during this beginning period--it will save your bacon if you get jumped.