Followup: Service manager agreed to replace my seatbelts for free if I'd clean them thoroughly first, to rule out gunk on the belts. I cleaned them, they still barely retracted, went back to the dealer, and a different service manager told me, "Sorry, but there's no way I can replace them. They work pretty-much as designed." It does seem like replacements would only help so much. It's the design that's screwy.

What if you cut about 2 or 3 feet off the top belt, and restitched it to the buckle? I bet that would function well, but you might have a tough time getting someone to stitch it.