on my disc trucker I have 17.5" thats a 22t granny and a 34t out back... but thats all just MTB 29er stuff on a touring bike... in reality the lowest I ever use around here is the 32/34 which is 25.4" and that is if i'm feeling almost dead I can climb it at 33.8" without issue as long as I push myself a bit.
on my road bike I've got the same gearing as you... i'm tossing a 34t up front (techincally it's out of range on the front Der but by all accounts it works just fine... out back i'll prob go with the tiagra 12-30t cassette... so my bottom end will be a 30.3"
you can do both fairly inexpensively if you're der can handle it... i've read of a number of people running shimmy short cage der with the 12-30t cassette...