Since my OP, I've upgraded my friend's PC from a Corsair CS550M (550 watt) to a EVGA G3 (850 watt) and that appeared to reduce stuttering by quite a bit, but didn't solve it entirely. It also helped a lot when I downclocked the GPU and Memory and turned up the fan profiles. For anyone else having this problem, here's a summary:
1. Remove all overclocks.
2. Always keep GPU and Memory voltages lower than stock. I'd recommend finding the lowest stable voltage, then adding 50 or 100 mv.
3. Keep temps below 75C. Around 70C is ideal.
The end settings on my friend's Reference Sapphire Vega 64 are in this range:
GPU Clocks: ~1300 Mhz P6, ~1400 Mhz P7
Memory Clocks: ~950 Mhz
GPU and Memory Voltage: 1000 mV
Fan Profile: Up to 3000 RPM, max temps 75C
My friend's card max overclock is:
GPU Clocks: ~1450 Mhz P6, ~1550 Mhz P7
Memory Clocks: ~1150 Mhz
GPU and Memory Voltage: 1050 mV
Fan Profile: Up to 4400 RPM, max temps 85C
So keep in mind the end settings that work for NMS are approximately a ~150 Mhz downclock from max OC on the GPU, ~200 Mhz downclock from max OC on the Memory, and 50 mV lower than max OC. I provide these numbers so you can get an idea of how your silicon should fare whether it's better or worse.
This probably made 80-90% of the stuttering go away on medium settings, 1440p. Definitely far from a great experience, but it at least matches or exceeds a RX480 and is tolerable.
Really wish that AMD would address this issue in-software rather than send NMS players on a witch hunt for the best settings! To be fair though, a lot of it has to do with NMS as other games run completely fine.
