A really good lens should give you crisp, razor-sharp images at 100% viewing--full size on the screen. If you have to scale the image down to make it look sharp, something is off, but before blaming the lens, you need to make sure it's not operator error! One likely culprit is a shutter-speed that's too slow, especially if you're going after a moving target. Also, for birds in flight, I recommend turning off the VC stabilization--it could actually be working against you. And if you've got a UV filter, try shooting without it. Some filters, especially less-expensive Best Buy type ones, can cause real image degradation. (I don't use 'em normally, and I only use really good ones when I do---others will cringe at this, but it's a personal choice.)
If you've got a fast shutter (at 300mm for a moving target, say 1/1000 of a second or faster--1/1600 is ideal; for a still subject at 300mm using the VC, maybe 1/800), no filter on, and the image is still soft or blurry at full-size viewing, then it might well be a poor copy of the lens. For what it's worth, zoom lenses are usually a tad less sharp than good prime lenses, but you should still be able to get usable 100% crops. Adding a 2x would only make things worse.
I hope it's not the lens, but if it is, you should be able to get a new one from Tamron.