I've never used a 2x t/con, only a 1.5x, that degraded the image quality quite significantly, it also reduced the effective maximum aperture by one stop. General consensus is, as Roy says, that a 2x will degrade image quality even more. You will also lose 2 stops of light with a 2x t/con, i.e. a lens which is normally wide open at f/5.6 becomes an equivalent f/11 lens. So, you can hopefully see you'll need a LOT of light to get an acceptable shutter speed, especially as you'd be using a "600mm" lens on a DX format dSLR - about 1/1000th would be needed for shake free images hand held.
You really are on a hiding to nothing using t/cons, of any magnification, with anything other than the very, very best quality lenses (ideally primes - not zooms). You're after a big, good quality lens on the cheap. I'm afraid if it was that easy, everybody would have one.
When I used my 1.5x I found it was actually just as satisfactory, if not better, to do a selective from on the image taken without using the t/con.
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