The issue continues to exist in the latest beta drivers, 15.11.1.
I finally figured out how to correct this. With the new crimson drivers, I can add custom resolutions for the lower resolutions that I want stretched. I think part of the issue is that my native refresh rate on the monitor is 75hz, so when the drivers try to find a matching resolution, it's probably not finding that refresh rate for some reason.
So I setup custom resolutions for 2560x1440 and 1600x1200 (DosBox games) with the refresh rate of 75hz. I picked a timing standard of CVT-Reduced Blanking, but it keeps getting replaced with CVT. Not sure if CVT is the correct pick or not, but it's working.