Mike, I'm not trying to make this an argument between venders.
However, most of the guys here roll around with 3.27-3.55-3.73 gears. You have the occasional Cobra with the deep rear gears, but remember that most of these cars either use transplanted BMW rearends or transplanted Ford 8.8 rearends. These guys with 3.27-3.31 gears would not benefit much, if any at all, from a .50:1 overdrive.
Now granted, most of the newer muscle cars from Ford/GM available with the T-56, such as the Mustang Cobra, Corvette, etc, have the higher rear gears with the additional .50:1 overdrive. I've had as much seat time in these as the next guy and shifting into 6th gear at anything below 75-80 mph is pointless.
To me, a T-56 is more of a bragging right than anything else....."...oh yeah, well my transmission has SIX speeds...."
With the added cost, size, and weight over a TKO 600, the benefits for the average street car and Cobra guys are negligible.
Again, I'm not trying to see which one of us can out-pee the other one, but like me, I'm sure you have the customer's best interest at heart, and in my opinion, this transmission (along with a regular T-56) is not the best match for a Cobra.