I did try it out many combinations in stores within a couple of hours driving distance before making the decision to go with the EF300 and the 2x tc to fill in on the long end. The 400 f5.6with the 1.4 TC on it is just soft enough in the center to make extreme cropping more of a challenge in PP than I want to deal with.

The other factors that entered in were losing AF and no IS in the 400. With changing to a 1D body, AF is retained. The difference in cost between lenses was going to shrink considerably if another body was purchased to go with it. In my area, 1D bodies seldom come up for sale used in a condition that would make me want to buy them. that 400 is lighter, but the 300 is still very handholdable and the IS is of great benefit.

There is very little effect on focus speeds with lenses I have when using the new TCs. Not sure how that holds up with older bodies, as I haven't kept any.