That's interesting - I had always thought I was limited to ~2K on ubermix as well, but during this exercise of finding a way to get to higher clocks, I found out I could get to ~2.1K w/ no timings changes. I guess the 2K limit (at least for me) was more the case under CN - ethash must be a bit easier on the mem.
I'm also surprised you can do REF 30 - I can do higher refs w/ CN, but for ethash, I'm seeing a couple percent errors already at REF 10 (def depends on ambient tho.)
Maybe we're using different ubermix versions? iirc, a couple different ones were included in PBE over time. I also recently noticed generaleramon had published later versions (even past 3.2,) one of which was suggested may be ideal for 2150.
I think I am running Ubermix 3.1 but it's been a while. Your original values for RC and RFC match mine. At REF 30, I have 1 bad hash in about 24 hours. On my Hynix cards, I can't run beyond REF 18 without too many bad hashes.
This reminds me of the days when a group of us using ATT6300 PC's (8086 machines) would lower the dram refresh rate from that set in BIOS to gain about 12% in processing speed. Of course, our CPUs were running at 8MHz so that was a big deal for us.