Ok, I did my own testing, just for kicks. Had to fully engineer another 7D thruster... I compared 6A, 7D and 7A thrusters. Same (explorer) Anaconda setup, all complete level 5 dirty drives with drag drives, all pips to engines.Turned out my memory did NOT serve me well, so the info about 7A and 7D performing the same is WRONG, sorry.
But even more surprisingly, 6A and 7A performed exactly the same! I mean EXACTLY, the turn rates, top speeds, all the same far beyond my certanty of time measurement. Did three tests for each with each thruster, so I thrust

my results. The only statistical difference is that with 6A my jump range is 72,79LYs, with 7A it is 68,69LYs.
7D is actually a bad choice. The jump range is 73,68LYs, so less than 1LY extra for lessened performance. Turn and roll times aren't much worse but top speed is. If anyone's interested, I can post all the results (turn, roll times...), though they only apply to my build, naturally.
EDIT: Tried testing fully engineered 5A thrusters, too, but I wasn't able to mount them. My guess is that in most cases a size lesser drives will perform exactly the same in explorer builds. Which means I have two more thrusters to exchange... Well, one at least. Will test this with Krait mk.II, too.
EDIT2: Just remembered I could compare the accelerations, too. At least the times needed for the ship to reach 100m/s and full speed, that should do the trick.