Get a MTB rear derailleur and a 11-32 or 11-34 9 speed cassette. That (along with Ultegra 6510 shifters) is what I run on my touring bike. For me, the larger gap between cogs (even while unloaded) is scarcely noticeable and I still can run a road cassette when I find I'm not using the lower gears. NB: I have a 46T big ring; with 50-53T road rings you might need to put a few more links in your chain.