so what is the optimal temperature for gpu's in a mining rig?
Nobody knows really, its all just speculation and anecdotal evidence from tiny sample sizes you can find here. Most recommend sub 70C, all we really know is where AMD/nvidia start throttling their cards so keep it below that at least.
as well as their risers sata cables?
You generally shouldn't use SATA powered risers. Sata power cables have to be designed to carry at least 54W while PCI-e slots are designed for minimum of 75W and it's a known fact that some graphics cards utilize all the power they can get from the slot (especially polaris based cards). That is unless the cables are thicker than their minimum required by spec, if they have AWG 22 or lower written on them then its fine (for a single connector). Molex powered risers are a safer bet though since they are good for at least 132W.
Now for the cable temperatures - they shouldn't go over body temperature if they are handling their load well, if they feel hot to the touch then you should reconfigure. Computer power cables generally have their weakest points around the connectors, a hot cable in the evening could mean a charred connector by the morning.
