Personally, my opinion is that anything beyond a 5-speed is extraneous in a musclecar. The only reason manufacturers are going with 6, 7, and even 8-speed transmissions is because it's the only way to keep engines in their "sweet spot", which gets to be rather peaky with multivalve engines, variable valve timing, etc. The automotive press bashed GM for a long time for only having a 4-speed auto behind the venerable 3.8L pushrod six, but that engine was a 2-valve pushrod motor with a broad torque band and it really didn't need more than 4 gears. Three lower gears, direct fourth, and an overdrive are plenty for a 2-valve musclecar engine. A 6-speed merely introduces an extra gear that, at highway speeds, lugs the engine and probably hurts fuel economy. 6th gear in most T56 transmissions is a second overdrive with a .5:1 final drive. You might make use of it at Bonneville, but at highway speeds it's probably not optimal for many of the cars with which we play. This isn't the case with the Richmond 6-speed, but it's got a pretty-sad torque rating.
My '73 Challenger got better fuel mileage with 4.10 gears and a 4-speed (around 17MPG) than my '73 Charger got with 3.23s and an awfulmatic (14MPG on a good day). Both cars were 340s, but the Challenger had the more-aggressive engine (early higher-compression pistons, '68 4-speed cam, etc.). A lot of factors come into play, like the fact that the Challenger was lighter, had slightly-larger tires, and the clutch doesn't slip like a converter, but even so the RPM difference at 60MPH was 800+ RPM between the two.
To be honest, I think the limitation lies within the carburetor and distributor more than anything. There's only so much one can do with metering rods, jets, power valves, advance canisters etc. and a lot of my theory flies right out the window with programmable MPFI and computer-controlled, where the fuel curve at cruise can be set lean with a ton of advance while the same curves at full throttle can be downright gnarly, all from the comfort of your laptop... hence the reason the Imperial will be MPFI with distributorless ignition, all controlled by the MegaSquirt I literally built on my kitchen table.