most likely there were 20 other things that made the difference.
I honestly cannot think of any way to test this idea.
It's pretty easy to see if DT's have an effect or not after dropping,just load the DT's and drop them and look to see if the hanger is still inplace. Then respawn without the DT's and check to see if the hangers are still attached,if they are then the DT's have no effect after they are dropped. If there are no hangers visable when you up without DT's then the DT's have an effect even after the dropping.
In the German plane the rack to hold the DT isn't on the plane if you don't use a DT,on the P51 as an example the DT hangers are always on the wing so the flight performance is the same whether you loaded DT's or not.
IMHO dropping 50 or 100 LB's of ammo make almost no difference on an A/C that weighs 5000 or more pounds. Fuel weights are another matter al together.