Put the 11-28 on and give it a whirl. That lowers your lowest gear by about 12%. If your crank can take a smaller chain ring you can get a little more by going to a 34 or even a 33 tooth small chain ring. Also, have someone knowledgeable look at your climbing technique, particularly when you are in the saddle, to make sure you are involving as many large muscles as you can (are you feeling it in your buttocks?).

I do most of my riding on my touring bike, so I have a triple (48-34-24, 11-28). However, I have never actually used the granny gear when unloaded. Well, I did use it once to tease a friend on a climb when he ran out of gear and was struggling to turn the cranks over.