The results in this R9 380 hot rodding thread are somewhat incomplete in that corresponding changes in power draw with mod's would also be of interest.
In my blog post the link includes 2 modded BIOS files with DPM 5, 6 & 7 under-volted: 380NR1100.ROM and 380NR1150.ROM
How much have you gotten out of those? I've passed 25MH/s with custom timings, but at that point it needs a little more on the core for it to keep up.
I've only been able to get 25Mh by pushing the memory over 1700, which hasn't been stable on any of my cards. I did try a couple custom tweaks (modifying individual values in the straps), but gave up after testing a couple values. If you're willing to share the strap decoding utility, then I could create custom straps based on what timings I know are important.