That PSU is of unknown reliability:
http://www.tomshardware.co.uk/answers/id-1804779/power-supply-unit-tier-list.html
And I found a similar thread here:
http://www.tomshardware.com/answers/id-2226253/year-cooler-master-thunder-500w-270x.html
I don't think you should go with 270X with that PSU, a 270 would be the best I'd want to try out with that PSU.
For a system using a single Radeon R9 270 graphics card AMD specifies a minimum of a 500 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 28 Amps or greater and have at least one 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
I don't see a cause of worry for that GPU in your PSU. The only one being quality, its really low quality, I'd rather upgrade PSU first and then save up sometime for GPU.