Being the large fella that I am, I'm switching to smaller front cranks plus a big-tooth cassette.
I can't find the link, but I think it's a thread somewhere on BF that gave me my plan.
In short, take an 11-34 cassette where the first 2 cogs are loose, order a 13t first-position cog and lock ring, I think Ultegra 6600. Take off the existing 11t and 13t cogs, and replace with the 13t first-position cog. Now you have a 13-34 with 9 cogs. Then add an e*thirteen 40t expander cog for Shimano. Now you have 13-40. 13 to 15 shift will not be optimized, but should still work OK. Not the simplest solution, but should work. I'm hoping it does. Sadly 22/40 is what I need to get up 6+% grades.
The other option similar to above I've seen is to replace the 15t and 17t on an 11-34 with a 16t, but then you have 11-13-16-19 and that might be too big of jumps, depending on your riding style.
This should also work with 9sp 11-34 cassette, as long as you get the spacing between the 34t and 40t rings correct, as it was designed for 10sp, but my understanding is the shift ramps are the same.
If I find the link to the thread I'm thinking of, I'll post it to this thread.