It's well known now that the very same GPU with the 16.4.1 drivers has a lower hash rate than with the 15.12 I absolutely don't have the beginning of an idea to why. Anyone?
That's pretty much impossible to answer. A driver is a hardware's soul, it has full control over the hardware and what it will do, and how it will do it.
A few things that could influence it:
- Different task scheduling, so maybe the GPU is doing something else with a higher priority than before, giving less resources to hashing.
- The driver might have a modified quality-of-service algorithm, maybe more aggressively taking care of overheating (e.g. lower its throughput faster as temperature increases).
- Different memory management that maybe does more copies in case of ethash, but less for some other task types.
- Last but not least, one version may contain a bug over the other that does some extra/useless operation that impacts performance.
All in all it's pretty much impossible to answer it properly without a deep understanding of both the hardware and the driver code itself too. Even if the driver works perfectly, it always comes down to balancing various things to achieve an optimal performance on some baseline benchmark, which may or may not lie close to the requirements of ethash.