A BIOS update solved the dual-channel configuration - it turns out I was running a rather early version of the BIOS for this board - and I re-ran PerformanceTest to get a fresh baseline. Unlike the original test I also ran this after a clean boot with only background tasks active, and I used RamMap to page out as much as possible (because PerformanceTest considers free physical RAM when calculating the memory score - which is bonkers, especially when having a SSD means lack of physical memory really isn't an issue even for memory-hungry programs).
Clean run, BIOS 1305
The result is a nice increase from 1619 to 1791, with improvement to everything bar disk (possibly that suffered from contention when paging stuff back in?). This is with the auto-detected RAM timings of 5-5-5-18 @ 400MHz 1.8V.
Next is to adjust the RAM timings. This was fairly straightforward - just drop into the BIOS, enable RAM overclocking and update the timings (leaving everything else on Auto). I also had to boost the voltage to 2.1V - to do that required setting Ai Overclocking to Manual and then overriding the DRAM voltage. And the result?
Clean run, BIOS 1305, RAM timings 4-4-4-12 @ 400MHz 2.1V
I'll take that 27 point increase, thankyou very much. I never expected a significant change and there's enough variation in the other numbers to largely hide this in the noise. It becomes a bit more interesting if I look specifically at the memory score:
Memory Mark with 5-5-5-18 @ 400MHz 1.8V
Memory Mark with 4-4-4-12 @ 400MHz 2.1V
That's nearly a 10% reduction in latency and a corresponding substantial increase in sustained memory bandwidth, and all technically without overclocking my system (although pleaseremove disputes the lack-of-overclocking, given that I've run Nyx with the slower timings for the best part of 10 years). Nice!