Hey guys, the 75mm 6MT that is coming in the new Forester is the same unit that comes in the Legacy, Outback, and diesel Forester.
The only version that will give the "overdrive " effect you're looking for is the diesel Forester versions. Every other iteration uses very similar 6th gear ratios to the normal 5th gear in previous gearboxes.
also, the 75mm 6mt is based on the 75mm 5mt. The STi gearbox is a completely different 85mm unit which uses larger diameter gears.
It has yet to be determined whether the 75mm 6mt is stronger than the 5mt. They're designed similarly using a split-case design, which isn't the strongest. The new gearbox has yet to be truly proven at higher power levels. The diesel has been a good test, amd the MY10 Legacy GT guys have only pushed as far as 400 lb-ft of torque. That specific LGT was sold off before they could conduct any durability tests on the gearbox.
I don't specifically know whether they retained the same gearbox length of the 5mt, but if they did then it will inherently be weaker than the 5mt since gears would have to be narrowed in order to fit everything into the transmission case.
Essentially though, if the gearbox still only has a 6th gear ratio of 0.7xx on a 4.111 final drive then it isn't going to be much more efficient on the highway than outgoing models. The extra gear allows them to space the other gears for better fuel economy.