Arguably, the Jug is the most weight-sensitive aircraft in the game, and generally, fighting it as light as possible in the MA should always be the goal. 8 guns with 425 rds/gun is a tremendous weight penalty, if you're intending on air-to-air. 6 guns with 425 rds/gun is heavier than 8 guns with 267 rds/gun. 6 guns with 267 rds/gun obviously decreases the wingloading and assists the roll rate, but as others have said before, its still a P-47 and heavy relative to other aircraft. Loosing the two guns seems like a high penalty for the performance gains.
For escort and fighter sweep missions, I only fly 8 guns, 267 rds/gun. You still have the snapshot of 8 guns, but almost 500 lbs lighter from the ammo difference. I usually run out of fuel before ammo, even in the Jug-N. Only if I'm rolling for air-to-ground or intercepting bombers will I consider the heavier ammo package.