I feel obliged to point out this thread:
It’s a bit of a rambling discussion, but worth skimming.
Key point is that you need a good PD power supply for higher current (eg faster) drives.
Also note that since we only have a single PCIe lane available that will be the real implacable bottleneck, though maybe the bus can be overclocked? 
Despite the bus limitation getting a ‘fast’ card with a lot of onboard DRAM cache could help a lot with responsiveness during random disk access, at the expense of drawing more power and being more vulnerable to PSU issues.