I just purchased an ultegra 11-28. That is probably the biggest rear cog you can run on a regular road derailleur unless you have a "mountain bike" derailleur or what some call a long cage derailleur. However, a 28 will not be a huge difference from the 25. Better, for sure, but not what I'd consider a bailout gear. I currently have a 27 and on steep sections I'm wishing for bigger. I don't think Shimano makes one, but Sram has an 11(or 12) 32 and a 36 option. You would probably need a new RD but that would be an option if you really wanted a large rear cog.