At a 6% grade, my cabrio has *no* problems at 65 in 3rd gear. It was tough keeping it in 4th gear at 70 in similar grades but 75 was okay for some of those steep grades, being further into the engine's power band. If you're shallower than the 6% grades I experienced going from Oregon to California and back, you should be good.
Sometimes kicking into manual to guarantee you'll stay in 4th for the power band is a good way to go, rather than letting 5th gear wimp you out of your desired speed. These conditions are where the manual mode really shines through!