I'm considering getting a 2.0 teleconverter for my 70-200 2.8. Other than going to 5.6 does it also reduce the iq? Thanks I'm still learning here.
1 A TC reduces the IQ systematically because it blows up the image of the naked lens by a factor of 2 (for a 2x converter you are considering) - lens flaws are scaled too. The upscaling by a factor of 2 means that the same light is distributed to a 4 times larger area reducing in 2 stops light reduction.
2 A TC introduces additional glas elements and reflects/scatters light. Modern zooms have 10-20 lens groups, the 4-6 lens groups of a TC will have some effect in this department, but it will be moderate.
3 Some TCs are co-optimized with one ore a few lenses. I am shure that Canon optimizes them for the 2.8 300 and the 2.8 400 lenses and that different TC series are optimized for the corresponding lens series. If you have an old 2.8 300 the 2x TC of the first series MIGHT match better.
I have the 2x TC version I (170 bucks 2nd hand) and it works well with my 4.0/70-200 L 4.0 non-IS and the 5.6/400. I decided to get the 5.6/400 because it's IQ is very fine from f/5.6 and I have a longer reach with TC.
Best you can do: Try your telezoom with a 2x TC (optimum: I + II + III series) and check the results for YOUR PHOTOGRAPHIC APPLICATION)
ADD (just seen): Kerry B's remarks about the 300 support my 3rd point!