The clarity of 1.4x (and 1.5x) are way better than 2x in my view.
There's no substitute for a nice bright aperture lens....a 500mm f/4 is certainly a cracker of an optic.
The shorter 300mm f/2.8 is more manageable, bolting on the 1.4x where neccessary. Ideal from my own point of view unless I did more cricket (or wildlife like you)...where I'd be looking at something longer.
I remember in the press a good few years ago there was some coverage on long/ bright lens designs.....optic designers were at that time exploring the liquid filled prototypes. One of the key advantages of this were massive savings in weight. i guess this idea was either ditched or put on the 'back burner' (??). Whilst i guess there's limitations on the physical properties of crystal/optical glass, using other media would blow traditional lens designs out of the water! Imagine a 500mm f/4 you could handhold as easily a 400mm f/5.6!!!
Anyone seen anything in the press about these designs?....last time I saw anything was in the mid' 1990's