Here are the two combos at their best settings, compared to THE reference (300/2.8 VR) at its worst aperture, which admittedly is almost equal to its best one ;-)
Ahem... as you see there's no substitute for REAL glass :-)

Anyway the TC-14E gives the 70-200 VR even more versatility with very nice image quality.
I never thought a TC worked so well on a zoom but now I won't worry when I'll have to use this combo.