For the Radeon R9 295X2, AMD did not adhere to the PCI Express Specifications for the 8-pin PCI-E connector. AMD is drawing 2.24 times the current defined in the specs for that connector.
The EVGA SuperNOVA 1000 G2 only has a single +12V rail. You can try drawing up to 83.3 Amps through the connector but the insulator jacket on the wiring would melt and burn before you reach that amount of current. The modular cables are 16AWG wiring which is good for up to 66 Amps per modular PCI-E cable.