NEVER use a SATA to PCIe connector - don't know who the hell thought of that idea but that is just asking for a problem since the SATA connectors were never designed to deliver that much power.
How much power is SATA cable rated for?
I have 5830's that's undervolted, and was thinking of using SATA to PCI-E power cables.
IMO, just use PCI-E plug splitters. Some power supplies split a set of 6 wires (3 hot, 3 ground) to 2 6+2 plugs already (300 watts!). With SATA, you only have 1 hot and 2 grounds, and they are likely thinner gauge wire, plus the connectors are not rated for a significant power draw.
Here are the PCI-E power plug splitters that I use. They take in a 6 pin plug (or 6+2, but it only takes in 6) and output 2 6+2 plugs, essentially adding one additional PCI-E plug per splitter. I love them. They are $3 each with free shipping if bought in volume at newegg.
http://www.newegg.com/Product/Product.aspx?Item=N82E16812198018