Watts are (roughly) current x voltage. In your case 45A x 12V = 540watts. More than enough.
The 6 pin connector is defined as 75 watts and the 8 pin is defined as 150W. Its just the way it is. All PCIe GPUs also pull power from the PCIe bus. Once that limit is passed you need auxillary power.
Yes you can add up all of the various wattages but the problem is the other devices dont report their consumption - so it doesnt really matter.
To either confuse or make this clearer, the onboard voltage regulators change our 12v to 1v (apprx) and pick up a bunch of amps. So our 75W input (12v x 6A) becomes 75A at 1V - but its all the same power....