I'm not sure on the mustang however I test this offline in planes I am curious about. For example if I take the 109k with dt, I drop it on the runway before firing the engines up, then take off, get to 200 ish alt and hit x flying out over the water. Once the speed is leveled out I look at the fuel loadout remaining (fuel computer on clipboard). I then mimic the setup without loading a dt and up and fly the same flight path. Once the computer shown fuel remaining is equal to that of the previous flight's topspeed I then have a comparison.
I know this isn't maneuverability exactly but that little bit of difference shown can sometimes be the difference when you might need that little bit of extra E (say to get the nose up for a shot...or being able to manuever effectively vs wallowing around). So in the 109k the topend is around 6 mph less if you load the dt and then drop it vs being clean to begin with. So in the 19K I generally just up 100% internal and no dt. In the Ki-84 I saw no difference so I frequently up 50 internal with dts.
I've always considered a turnfight to be just as much of an E fight as a any other style of encounter. I have (maybe wrongly) associated top speed comparisons of the same fighter in different configurations as being relative to the ability to gain or retain E in those different configurations, so that's what the point of those tests are for me.
My 2 cents.....
I suspect that it's not really that important given all the different variables you would be encountering in a fight that are far more likely to effect the fight. For example, encountering an nme soon after upping with 100% fuel load against a plane that has flown a full sector to get to your position....or the most common variable...encountering an unlike plane. I think what would be more important given those circumstances is recognizing your weakness and your strengths relative to your opponent and trying exploit the good stuff and avoid the bad stuff and using solid acm. While turning with everything you see in a mustang may be looked at as violating the last sentence in terms of comparitive weakness's and strengths with many commonly flown planes, I think it's fantastic training for learning and using acm. I do it all the time while trying to learn the 109K and Ki-84, but I die alot too
