As long as there are enough system resources, HDD, CPU, RAM and PCI-E lane speed makes very little to no difference in hash rate. Almost any system will work. The biggest diffrence is how many cards you can get to work without a ton of grief. Core clock has a big effect in hash rate. Even more so for compute intensive algorithms like ZEC compared to memory dependent algorithms like ETH. Also every card has a different ASIC value from the factory which determines how well and easily the card overclocks. You can check the ASIC value with GPU-Z.
That's what I thought however there was a clear reduction of 10% hashrate
So how do you reduce power draw if you cant underclock the GPU core?