Unless you're an astronomer or in the espionage business, there's very little real-world utility to lenses longer than about 800 mm. The most common case would be birding...but a successful bird photographer knows how to get close enough to the subjects to use a "normal" supertelephoto.
The 300 f/4 is an excellent lens. It's a bit short and slow for birding; if you'd have a teleconverter permanently attached, strongly consider the 400 f/5.6 instead.
You're always going to get the best results with a larger sensor and a (quality) lens that's (natively) long enough to properly frame your subject. But, if you're distance-limited...well, at first, it's a tossup between a (quality) 1.4x teleconverter and an APS-C body with higher pixel density. Some combinations will be better than others. If you're still distance-limited in such a situation, your only choice left is both. And if you still need to significantly crop with a teleconverter on a crop body, then either you need to get closer to your subject or you need better equipment or you're in the worng line of work.
Of course, what you're doing with the final output is a question, as well. If all you're looking for is something to post on BookFace, then you can settle for a lot less than if you're on assignment for <i>National Geographic.</i>