I expect that it would be so-so as far as the wire is concerned. PCI-E specs call for 75W/6-pin connector, and that's spread across three 12V lines, so you wind up with a maximum of a little over 2A/wire, which isn't a huge deal. The bigger question might be the PSU's rail distribution and connector construction. Safest bet might be to use a single-rail PSU, take power off two of the PCI-E cables, and then join them near end, for the part that's actaully visible.
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