Problem with the several 6-speeds I've driven (only owned 1 but have driven a half dozen-ish) is that the top end feels about the same.. Similar RPM at highway speeds.. Seems to drive like you get 6 gears that cover the same low-to-high gear ratios instead of 5. I'm happier winding out 3rd a bit longer and skipping the extra shift.
Now, if you want to build a real "overdrive" tranny that runs 1,500 - 1,800 RPM at 70 MPH in 6th gear, I'd be interested in test driving that. Sure, you'd have to downshift to pass, but, that would be some great fuel economy and a faster top speed (although you'd get there more slowly). 1 thru 5 for normal driving and a legit overdrive gear for highway with the cruise control on.