I love steel, so I'd be sorely tempted to upgrade it. That said, I still ride a bike with 7-speed on it. My "racer bike" has 9-speed, and I don't feel inclined to switch it to 10.

What do you mean by "outgrowing the gearing"? You've got some pretty big gears available already, and I'm not sure that a switch to 10-speed will change that much. You'll get some closer gearing in a 10-speed cassette, but the cassette range should be about the same.