Update time! All components are here including the power supply shim. It look great!
It's 6mm thick aluminum. The width allows standard 1/2 inch screws to be used to attach to the PSU and the PSU/Case's supplied 1/4 inch screws to attach the case to the plate. The countersunk holes are unthreaded since the PSU holes are threaded. A small amount of hand filing was needed around the switch and IEC socket, I was off by a mm in my measurements.
Fully mounted I can see a few tweaks I'd make if I had to redo the plate, but it's definitely functional.
Finally the money shot, Can I plug in the SATA cables?
Yes, yes, I can. I only have two hard drives so only needed two SATA connectors, but hadn't thought about the mounting plate being a clearance issue with the top SATA connectors. Oops. It'd be easily fixable with a mill, saw, or file if I needed those ports in the future. ?
Onto the next issue, the ID-Cooling IS-60 CPU cooler won't fit on this motherboard. The large chipset heatsinks are too tall. I'll have to saw off just under a centimeter of fins from each side of the cooler. Thankfully there's plenty of room to do that.