I think losing top speed is a bit of a myth about gears. I was able to run to 140 mph (GPS verified) with 4:10's. Somehow I don't think the car would go much faster if I had the original 3:27's still in. What I believe happens to maintain the same top speed is that the taller gears enable you to run to a higher engine rpm than you could with shorter gears. For example, the AOD-E in 4th and at 5000 rpm your speed will be 138 mph whereas if you had 3:27's and were able to reach 5000 rpm your speed would be 173 mph, not very likely. You would reach the 138 mph though at 4000 rpm and at 4500 rpm your speed would be 156 mph if you could get the engine speed to that level.
I dont see how it could be a myth. I mean, thats what they do. Maybe they allow you to get to a tops speed that you couldnt get to with stock gears because of the distance you had to have to get it. But you redline sooner in 5th with higher gears. You cannot go as fast. And when I say top speed, Im more talking about the speed that the gears can take with the motor redlining. Im not taking HP, aerodynamics, ****ty Mustang suspension, etc into effect.
All I know is that I hit ~145 on the speedo with my stock 2.73's, and it winds out WAY too much with my 3.73's to try it again.