Thanks for the advice. You're right; there is less drag in the middle than there is at either end of the gear range. I'll chalk the rest up to the surprisingly strong return spring on the stock derailleur.

One note pertaining to drop bars and bar-end shifters: I'm using an old touring one, and it won't go through the whole range of the cluster, even though it worked fine with 8/9 speed shimano derailleurs on my other bike. I think this has to do with the "1:1" actuation ratio on the sram derailleurs-- it takes more cable pull to make a shift with the sram. If you want to use a bar-end shifter with the stock derailleur, you'll have to use it in friction mode, and possibly grind off some material to make it cover the whole range.