I'd say the biggest advantage if you take out memory access is not latency (since most of the time latency gets hidden via pipelining), but power consumption.
When developing kernels the amount of times memory is accessed has huge impact on power-efficiency, less on actual computational throughput (especially on GPUs), unless you hit bandwidth limitations (which right now Ethash does).
EDIT: NVM - you're asking about the comparison of FNV1 vs Siphash. I haven't analyzed Siphash but I can say that FNV1 is exactly 5 clock cycles on a modern GPU (4 for a 32 bit multiply and 1 for an XOR).
When developing kernels the amount of times memory is accessed has huge impact on power-efficiency, less on actual computational throughput (especially on GPUs), unless you hit bandwidth limitations (which right now Ethash does).
EDIT: NVM - you're asking about the comparison of FNV1 vs Siphash. I haven't analyzed Siphash but I can say that FNV1 is exactly 5 clock cycles on a modern GPU (4 for a 32 bit multiply and 1 for an XOR).