There is pretty much no
There is pretty much no benefit to SATA III hard-drives. It is a marketing thing. Hard-drives are nowhere near saturating the SATA II bus, let alone SATA III. Maybe in a very busy RAID0 config or busy multi-disk config with lots of simultaneous access… but not in an HTPC I don’t think.
The latest gen of SSDs do benefit and the last gen of SSDs should have benefited if the SSDs were in RAID0 but that’s about it.
Modular PSUs are a give and take. On the one hand you have less unused cables running about, but on the other the plugs into the PSU generally take up more room so if your case is tight (mine was) it is kind of a wash. I kinda wish Seasonic had some fanless, non-modular power supplies. Or just PSUs that were a bit more shallow (an extra inch to spare between the PSU and OD would have been nice in my Vidabox vCase3).