You may think you can, but you can't. First off, if you've got 1000 rounds of 20mm you have the belly pack. That's adding 600lbs and a LOT of drag, and if you fire off all the ammo it's still about 400lbs and a lot of drag. Either use the guns or don't take 'em.
Second, the weight of the 110G is about 16,000lbs (17,500 with full gas) Firing off ALL the 20mm rounds (750 basic package) only saves you 363lbs of weight. You're STILL in a slow, sluggish, 15,000lb airframe.
If you kill a spit it's one thing, but firing off your 20mms to save weight isn't going to make you ANY better than if you still had them. It's a drop in the bucket.