No the empty weights would be the same as with the Q. Yes, engine is the same. Given that the empty weight is the same and the nose guns are the same, the only major difference is the wing guns.
Trigger, your math is nonsense, because the 50cal gunpack only comes with 300 rounds per gun, 600 total. The 4x 30cal internal guns setup has 1000 rounds per gun, 4000 total.
That's where the weight difference comes into play.
The ammo alone for the 2 guns packages leaves the 4x 30cal package 152lbs heavier from just the bullets alone.
And as far as aiming goes: Assuming YOUR skill doesn't change, assuming you can fly the plane to get hits on target, any hits you land with 4x50cal are more powerful than those you land with 2x50cal and 4x30cal. That's just a fact of how AH is modeled. On top of that, the round are able to do catastrophic damage at MUCH longer ranges. 30cals become almost useless outside 200 yards, 50cals are good up to 400 or so. Assuming your rounds hit where you want, one guns package is clearly better than the other, weighs less, hits harder, hits further.