Thought I'd post an update on this, for those who may be interested...

I took the RB-T for its first long-ish ride last weekend (70 miles one way, then 70 miles home two days later). Everything went superbly, with the exception of fatigue in the part of me where my neck meets my shoulders. "Oh no," I thought. "The new stem didn't have the effect I was hoping for; this frame really is too big; I'm never going to be able to make it work; buyer's remorse wah wah wah" and so on.

Then I remembered that several years ago, when I bought my first road bike, I was feeling pain in the exact same part of my body. BUT that bike was closer to the correct size for me (52 cm). So I figured that if the RB-T hurt in the exact same place, the problem wasn't the frame size.

THEN I remembered that I took that road bike to an LBS for a fitting, and the woman who fit me determined that the handlebars were too wide for me. I had the bars switched out, she made a few other adjustments, and the pain went away.

So I measured the RB-T's bars, and they were 4 cm wider than the bars on my road bike. I took the bars off of the RB-T and replaced them with a (significantly narrower) pair of SR World Randonneur bars that I had lying around. So far, so good on the couple of short rides that I've taken with the new bars. I guess the real test will come when I take it on it's NEXT long ride, though

Happy riding.