It's not a matter of what the modems were capable of, it's a matter of what the phone line would and could actually carry. Maybe it would be different in a big city, but I don't think I ever saw anybody get over 33.6 before upgrading to DSL.
It did depend on line quality, though. We had some kinds of splitters and internal cabling in the house for allowing multiple phones (and eventually the modem), and I remember that prior to some changes made to that, I only used to get up to ~40k.