It becomes confusing when english is not the first language for everybody and some people starts doing math comparations. And from what Nicolas is saying, K-30 with 300mm f4 and the same ISO and shutter speed as D610 can get the same results (with the aperture being set at f5.6 on D610).
Biz- engineer, forget the Af for a second. Let's presume that Pentax Af is as good as D750's. The image quality from K3 can not be similar with the image quality from D750 if you use the settings recommended by Nicholas, those being:
Pentax K3 with 300mm @f4, ISO 800, 1/400
Nikon D750 with 450mm @ f5,6 and ISO 800, 1/400
Not to mention if you use these settings on K-30 vs D750 or D610.
But I think somewhere I misunderstood something, or Nicolas didn't do some part of his math calculations right.