The very thin wires in the extenders do cause a bit more current to flow because of their added resistance. Rule of thumb is any and all dual-gpu cards with extenders should have molexes as well, and single gpu cards should be OK without them. However, the 7970 is a beast that needs more power, so it might be a good idea to have molexes, especially if you plan on overclocking.
Actually the exact opposite is true. The PCIe spec limits current on the slot to 75W but most high end cards (those with 8pin PCIe power connectors) draw much less. I measured wattage at ~30W for a 5970 across the extender. A 5870 draws ~30W too.
A 26 gauge wire can safely handle 2.2 amps of current.
A 30 gauge wire can safely handle 0.8 amps of current.
75W / 4 wires / 12V = 1.56A per wire.
30W / 4 wires / 12V = 0.625A per wire.
@ 30 gauge, 1 foot, 0.625A the voltage drop is 0.132V which is within ATX spec.
Personally I would stick w/ 26 gauge wiring and max length of 0.5 feet but even longer thinner wires should be fine.So why is a single card worse?
4 GPU 60W total 120W total
6 GPU 90W total 180W total
8 GPU 120W total 240W total
The issue isn't the extender but the aggregate draw on the MB. To be compliant a PCIe 1x device must draw <25W and a PCIe 16x device which identifies it self as high current drawless than 75W. All devices must draw less than 10W until interrogated by PCIe controller (at boot).
It really comes down to how aggressive MB designer was in power distribution. Take a board with 4 PCIe x16 slots & 1 PCIe x1 slot. By the spec the MB should be able to have 325W worth of compliant devices however to save money the design may assume that only 200W or 150W or only 100W will be drawn from the PCIe bus. The higher the load the higher the risk you bump into the corners cut by MB supplier.Granted the shouldn't cut corners but the number of users who pull 300W+ across the PCIe bus are in a rounding error and beefy power distribution is expensive for a low margin product.
It gets worse when using a board with lots of PCIe 1x slots. The more PCIe 1x slots less power the designer may anticipate as a realistic scenario as PCIe 1x slots can only draw <25W and many (SATA controllers, USB3.0, sound cards, etc) draw <10W. (This is why the presence detect pin needs to be shorted on some boards for stability).
Sadly a lot of boards are non-compliant and available power across PCIe bus isn't a stat provided. This makes it hard to predict how much of a load the board can handle safely and efficiently.
As a rule of thumb I would be more concerned about overloading the board when:
1) Using more than 4 SLOTS (# of GPUs doesn't matter).
2) The board has less PCIe 16x slots and more PCIe 1x slots.
3) The board only has a 4 pin 12V connector not the higher current 8 pin 12V connector indicating a possible design choice in power handling.
4) The board is more of a budget model (not crossfire certified, etc).