Finally I found time to test my 5D3 with a 300mm f/4 (non-iS) lens with EF2X II teleconverter.More information about "not much light"
The test was inside my house and not much light.
It worked fine
Shutter: 1/50 Aperture: f/8 ISO: 25600
According to an EV calculator I found it worked fine to 3.6EV.
Next step would probably be to use half the lights I used...