Wish there was an accurate way of measuring power draw on the PCI-E slot. I honestly don't think it's very much. If anything, it powers the fan and probably some of the basic IC's. The PCI-E power plugs powers the GPU itself.
Also *shrug* I use unpowered risers on my 4x 5970 rig as well as on my lesser rigs. I've never had a problem. The worst thing I did was melt a molex cable because I powered 2 5850's at stock volts through a single molex line (melted the modular power plug on the power supply. The power supply still works
). The motherboard is also 100% unharmed and fully functional.
You MAY need to short pci-e presence pins. It's hit and miss if a motherboard needs this, but it always works unless the manufacturer explicitly states that certain slots are turned off when others are populated, such as my MSI Z77A-G45 (7 PCI-E slots. PCIE_3 turns off when PCIE_4 is populated)