While FSP is a proper brand (they make units for big companies like Dell or HP, manufacture for 3rd parties and also sell under their own brand), 300W is really pushing it (I have a 350W unit that can deliver 336W over the 12V rails, but it's still not a big unit)
Also agree on the Molex vs SATA for the adapter. SATA allows for 1.5A per pin or 4.5A total (cause it uses 3 pins per rail) that still gives only 108W over the 12V line, even with a 2to1 cable, Molex is rated for 11A (264W), which is enough for the 150W on the PCIe plug.
Still, having to use an adapter means the PSU is not build for that kind of load, which is why the PCIe plugs are typically omitted on smaller units.
The GTX 960 is rated for 120W TDP.
Add your CPU (lets assume something typical, so 80W), motherboard (maybe another 40W), RAM (lets say 20W), drives (take around 8W per drive) and even with the most careful setup that's another 140-150W, which brings the system to 260-270W, which is around 85-90% of the total 300W rating, which is split among the different voltages.
After checking the unit, you're limited to 240W on the 12V rails, which are 8A and 13A (which is and 96W and 156W) so you can't even be sure that you'll have the 120W (plus what your drives use) available for the GPU.
Looks like it's coming from an acer prebuild system.