Should I go back to OEM size tires?

Personally, I would keep the 265s. If you think you're really hurting for the slight amount of extra grunt, go with the 245s. I run a 285 on my pickup and while the big mountain passes require downshifting, compared to 20 years ago, trucks have so much horsepower that I don't think it even matters that much. I am passing semis on every mountain pass anyway, what's another few percent of output? If you don't venture off the pavement much, the 245s are better, and skinnier tires are better in snow and ice. On wet or dry surfaces though, you'll notice more traction with the wider tires.