Using 2x SATA->6pin PCIE and 2x Mini-6pin -> 8pin PCIE should be save even for most powerful GPUs. If I recall correctly, a single SATA connector is supposed to provide up to ~50W.
That might be somewhat OK for separate
SATA connectors, but Apple's are two connectors on a single line, meaning the full current draw for both would come from the same line
. And since PCIe 6-pin connectors are meant for 75w, not 50w, that's an additional 50w over the 100w the lines are designed for. At full load that'd be like pulling a Polaris 480 before it got the PCIe current limit fix through the lines. The PSU itself can handle it. Probably not two of these GPUs, but at least one easily. It's the lines themselves I'd be worried about. The GPU will draw whatever it draws - it's rated for that. I wouldn't want any optical drive (or anything at all really) on the same SATA line as the one I'm using to power the GPU. I sure as hell wouldn't want that kind of power surge going through the same line as any other equipment.
I guess we'll find out soon enough from Rommy whether or not it's stable for long term use.