Thanks for the replies..
First, i must confess that i don't have a Cobra. I have blown '93 Mustang GT with all the goodies one can almost imagine. I found that the conversations and advise in this forum more knowledgeable (intelligent?) than the other Mustang boards.
After a few more weeks driving with the TKO-II, I've learned to love it. The extra power in 5th gear is unbeatable and it makes my car sound more "angry" on the freeway.
My Mustang is already probably one of the most gas guzzling vehicles on the road getting only about 156 miles per full tank of gas. With the stock T-5 tranny, i was getting 175 miles per full tank. Gas is not much of a concern.
At 70 mph, i'm reving at around 3200 rpm but i think my speedo is off somewhat. My RPM gauge and speedometer travel almost in unison. So when driving at 120 mph, the engine is reving a little over 5000 rpm whereas a 0.64 5th gear will be just shy of 4000 rpm. A 0.64 5th gear at 5000 rpm would be about 150 mph. Approx 30 mph difference between the 2 gears! Too much high end is being cut. Not good when your up against a C6 Corvette.
Incidently, i beat a C6 Vette in open highway 1 week before my stock T-5 broke after 11-years! It served me well. After having the TKO-II installed, i noticed the change in 5th gear hence my concern for high-end speed.
I thought of changing the rear end to 3.55 but after figuring that i will lose low-end zip & acceleration. I'll pass on that.
The thought of "balance" plays a profound role in this case. 0.83 5th gear = more response and power but at the cost of high-end. 0.64 5th gear = more high end but at the cost of mid-range response thus the need to go to 4th gear when acceleration is needed.
Here's a thought: granted that my RIMs are the original 16' inch pony rims but with 245/50 R16 tires, would upgrading to a 17 inch help? How about going to a 17 inch rim (wider tire) and a 3.55 rear end?