Judging based on your specific situation, I think you should be fine. If your PSU has two 6-pin PCIE connectors in serial coming out of the same connector on your PSU (I'm guessing the PSU is modular, or else you wouldn't be able to see the number of pins on the PSU side), that just means the PSU is specifically designed to handle this load. It very likely has a single +12V "rail" with an extremely high amperage rating, or two very high amperage rails, but not 3+. PSUs that have more rails have less amperage per-rail. They wouldn't ship with such a connector, unless the wattage on the PSU was very high and each rail could carry a large current load.
What are the potential risks of using an improper power configuration for my graphics card?
It depends on the design of your PSU and your GPU. In the ideal case, your GPU or PSU (or both) would simply refuse to work, or power itself off, when it over-draws on the current, thus protecting the boards from damage. Attempting to over-draw can cause heat buildup, and in the worst case, fire, because amperage ratings are based on what the wires are rated to handle, based on their resistance (which is another way of saying "heat"). More current, more heat. So you need a lower resistance wire or a good heat dissipation strategy to carry a larger amount of current.
Of course, to be on the absolutely safe side, I am required to tell you that any improper hardware configuration could potentially lead to explosions, death, and loss of life, limb, or property, with no limitation (worst case, your computer catches on fire and causes your entire house to burn down, and then your house catches other houses on fire in the neighborhood, creating an enormous conflagration and billions of dollars of damage.... perhaps even the deaths of firefighters and residents.... but this kind of thing doesn't happen so much these days because of fire codes and electrical wiring standards, etc.)
That said, the only way to know if your PSU can bear your GPU's load is:
- Look up your GPU's Thermal Design Power (TDP) or the amount of power required on the +12V "rail". If you get TDP in watts, you'll have to convert this into amps @ 12V using the equation
P = V*I (P = power in Watts, V = voltage in Volts, I = current in Amps). Using some basic algebra, if you have watts (assuming 12V connection, which is absolutely the GPU standard for many years) and you want amps, the equation is
I = P/V, so divide the Watts by 12 to get the Amps required on the +12V rail.
- Look up your PSU's specifications and determine how many rails it has. Fewer rails will tend to have higher amps per rail; more rails will tend to have the total wattage of the PSU split up more, meaning that you'll have to be more careful not to overload one rail.
Now... here is an example
If you have a GPU with a maximum TDP of 120 Watts, 120W/12V = 10 Amps, using the formula.
Whenever you get your GPU's TDP, you need to subtract 75 Watts from it, because the PCI-Express slot itself (the slot you connect it to on the motherboard) will supply it that much power, per Wikipedia:
They can use up to 75 W (3.3 V/3 A + 12 V/5.5 A), ...
So 120 W - 75 W = 45 W. This is how much power this hypothetical GPU with a TDP of 120W would need to draw from the external power connectors (the 6- and/or 8-pin connectors you plug into the card after you have installed it into the slot). Now going back to amps, 45W / 12V = 3.75 Amps. That's not a lot.
If you have a PSU with two +12V rails, it's likely that the external PCI-E connectors are on a dedicated rail of their own, separate from the one that powers the motherboard's +12V.
If you have a PSU with one +12V rail, shared by the mobo and the PCIE connectors, all you have to do is make sure that the sum-total of the motherboard+CPU+disks+GPU+...(whatever else uses +12V) amperage is less than the amount rated by the rail.
Now, per TomsHardware, the R9-270X has a TDP of 180W.
Docking 75W by PCIE slot, that means the external connectors have to push at least 105W, or 8.75 Amps, in addition to whatever else might be bearing on that rail.
If you're lucky, if you have a modular PSU with more than one rail, you may be able to find a "rail map" that tells you exactly which pins on the PSU modular interface map to which rails. If you're unlucky, you won't be able to find such a diagram, and you'll just have to hope for the best :) Have your fire extinguisher ready! (just kidding, it's probably fine lol.)