the 13t diff on my brifter bike works well (26t to 39t) and was worth it to have to go up two gears at the back when you go down to the 26 granny from the mid 39, and if I wanted to go up three it is just 3 quick stabs at the brifter.

the 16t diff on my old dt shifter bike often had me shifting up three gears at the back, but usually two worked well when loaded because the bike would slow down enough that two was just fine--I would say that for unloaded riding the 16 is a lot--but then again, thats what tons of road bikes have with their 50/34 so I guess you can take that with a grain of salt (but these bikes are much much lighter than even a naked tourer, so its much easier than on a heavier bike)

really what it comes down to is how putting on a smaller granny is a quick easy way to get your gearing down, and for me the compromise is ok because for a lot of the time Im in the mid chainring and for climbs will often have a bunch of gears in the granny so Im in that for a while anyway. An aside, this is one thing that is nice having a 9 speed vs a 7 is that there are a couple more useable gears in a given chainring. Having ridden 7 speed for a long time, then 8 , then 9, as long as drivetrains are reliable and last a decent amount of time (mine always do) I'll take more gears in back, nicer to have smaller percentage jumps between shifts, pure and simple.

to the original poster, go to sheldon browns gear calculator and do a few charts for your bike with diff grannies and diff cassettes. Its nice to see the percentage between each shift, as well of course seeing what gear inches you have now and what a given change will lower your low gear. Just put in "gear inches" and your wheel size and then plug in diff stuff, do screen shots of them and save them so you can compare.