Found the actual specs tonight.

 Molex connectors (technically AMP Mate-N-Lock but they got commonly MIScalled Molex a long time back) are rated for 13 amps per pin - one pin on 12V (and 2 ground pins, though one is really intended for use on the 5V connection) gives 156 watts power capacity, PLENTY to power the PCI-E bus on a riser with lots of capacity to spare - the limit is usually going to be the WIRING.

 SATA power connectors have 3 12VDC pins (and 5 ground pins) but each pin is only rated 1.5 amps, so 4.5 TOTAL amps at 12vdc = 54 watts WHICH IS NOT ENOUGH TO HANDLE THE 75 WATT RATED POWER DRAW FOR THE PCI-E BUS SAFELY.
 Forget the wiring, the connector ITSELF is not rated for the power load needed for safe operation.

 PCI-E 6-pin connectors are rated 75 watts by the PCI-E spec but they're actually good for 8 amps per pin and at least 2 are connected to 12VDC (the third is sometimes used for a "sense" line), so even 2 pins 8 amps each = 16 amps at 12VDC = 192 watts.
 PCI-E rates the power connectors VERY VERY conservatively - but again, be cautious about the wiring which often is NOT that far overrated.