Is the wife going to be using the lens too, or is it just for you? She might prefer the smaller package. And, have you thought about just selling the 70-300 and getting a 1.4 tele to use with the 70-200? Some don't like it because of the way the converter attaches, but it would get you about 280mm with a better f-stop and have money in the bank. I don't have a good "cheap" way to get to 400mm besides another teleconverter, a cheaper crop camera tied to the current lens you have for the appearance of a longer length, or the swap you were talking about. The problem with the swap is that all you're getting is an older, bigger, heavier lens, and an extra 100mm. The bad thing about 400mm is it's not 500mm and that's what you'll be looking at next