When I first bought my Canon 2 x Mk3 extender I compared it (+ my 300 F2.8 L IS Mk1) directly to my then 600 F4 L IS Mk1. I was shooting at shortish ranges of about 6 to 10 meters (the sort of range I normally use for small birds) and, given good light, the 300 F2.8 Mk1 + 2 x Mk3 gives up surprisingly little to the 600 F4 Mk1.
Naturally the AF will suffer with the extender but, whilst a bit slower, it is still accurate.
Attached is one of the first couple of images I took with this combination on my 1D4 (didn't have the 1DX then). This is just the RAW file JPEG'd and scaled, no processing.
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Life is for living, cameras are to capture it (one day I will learn how!).