Sounds like I could probably make a 30t work with my old 105 derailleur. I'll try it with a 28 and then order a 30 (any recommendations for loose cogs?) if it looks like there's still some room to play with. The cassette will then be 14-15-16-17-18-19-21-23-28, with an option to try -21-25-30.

FastJake: Thanks for the chainring info. I would love a 40t (even in steel), but Biopace rings of any sort are increasingly hard to come by. I wish somebody would start making similar chainrings again.