However the 70-300 is no where near as sharp as the 70-200.
In what parallel universe? Unless you're talking about the 70-300 non-L, in which case, certainly. But, the OP has the 70-200/2.8 IS MkI, and the 70-300mm L is a fair match for sharpness across the range when the 70-200 is stopped down to comparable apertures (and with both wide open, the 70-300L is sharper).
He currently has a 70-200 F2.8 IS which is a magnificent lens. It works well with both the 1.4x and 2.0x tc and will autofocus with any camera with either converter giving a usable range of 70-400mm
Allow me to edit for content: He currently has a 70-200 F2.8 IS which is a
magnificent very good
lens. It works
well sort of ok
the 1.4x and delivers soft images with the
2.0x tc and will autofocus with any camera with either converter giving a usable range of 70-
400mm280mm, or 70-400mm if you have a high tolerance for mushy images
If the OP had the 70-200/2.8 IS MkII
, it would be a different story. But the MkI is the least sharp of the three 70-200/2.8 zooms (and the f/4 IS is sharper, as well), and the MkI does not handle teleconverters well at all.
For example - you can see that comparing
70-200/2.8 IS at 280mm f/5.6 (stopped down) to the 70-300 L at 300mm f/5.6, the latter is sharper. As for the 2x TC, here's what that does to IQ
. Hmmm...looking at that, the effect in the center is not as bad as I anticipated - but that's because the bare lens at 200mm f/2.8 is not that good in the center (at least, when compared to the MkII
So, compared to the 70-200/2.8 IS, the 70-300L would be an IQ upgrade if the OP needs the 200-300mm range, but the tradeoff is a loss of one stop of light.