I believe it is. I have many people try ditching ammo from rear gunner of 110, stating that it would help the maneuver, go faster etc. But I don't really know if it works. Some say it does, some don't.
It may help, but only minimally. Correct management of fuel/engagement regimes is much more important though. Fuel weighs much more than bullets. This does change, though, if you are in lighter planes and the percentage ammunition weight versus the aircraft total weight is higher, such as in the 109.
I personally hardly ever fly with 100% internal, unless in Tempest or other gas hogs. I plan out my engagements based on the weighting of my aircraft, therefore knowing how my maneuverability will suffer. I'll generally take 50% or under, and carry drops until I find a fight I wish to engage in, thereby giving myself maximum maneuver capabilities in any flight regime.