Hi Loren, glad you are enjoying the 400 DO!!...join the (small??) club!!
I use a mk3 2x as I also have a mk2 300 2.8, and I'm using a 1Dx which I think (although it's a bit subjective) improves the keeper rate over my previous 1 Ds3....although..oops..just seen you have a 5D3..
My photography is mostly wildlife, mammals, so I'm not doing birds in flight generally therefore perhaps not pushing the combo as much as others as my subjects tend to be slower moving, however almost all shots are sharp as a tack.
I'd go for the mk3 anyway, as firstly, it's future proof, and secondly, I think it's sharper than the mk2.
Best of luck!!
George.