My experience differs.
I agree that (as one example) a 400/2.8 Lens will out-perform a 70 to 200 + x2.0 Extender, but, I think that it is important to reckon what 70 to 200 lens and what Extender is being used.
From the gear-list provided it is most likely that the 70 t0 200 is the obvious lens to which an Extender be added. I have used Nikon. The 70 to 200/2.8 VR is a very good lens. It is unclear if that is a Kenko x1.4 Extender in the gear list or not. My experience is that Nikon Extenders are better than Kenko, however I am not sure IF a x2.0 Nikon Extender is compatible with that particular Nikon 70 to 200 lens and the Camera Bodies in the gear list: these might be factors in not wanting to extend longer than x1.4, but choosing to crop the image instead.
As we are comparing experiences and especially for the readership interested in Canon Gear: note that the Canon Extenders EF: x1.4 MkII and MkIII; and x2.0 MkII and MkIII are compatible with the three (3) EF 70 to 200F/2.8 L Series Lenses. The best pairing being the EF 70 to 200 F/2.8 L IS MkII USM and the Extenders EF MkIII.
As a general point nonspecific to any brand of camera or lens - Post Production is an important phase in the representation of the final image, and because the optic of any Extender will result in a softer image the PP of these Image Files requires a different PP technique be employed than what otherwise would be the normal workflow for an Image File made with the ‘naked lens’.
Apropos - Reduction of AF Acquisition Time and EF Zoom Lenses:
It being STATED by the Manufacturer that there is compatibility apropos AF Functionality is the first consideration. Amongst the small group of
EF Zoom Lenses which ARE compatible with Extenders EF in regard to AF when used with MANY EOS Bodies,
the three 70 to 200/2.8 lenses. In my experience, all perform very good to excellent.
Apropos Reports and Lab Testing of AF Functionality:
Several notable reports reckon that the slower AF using the EF70 to 200 F/2.8L USM; the EF70 to 200 F/2.8L IS USM; the EF 70 to 200F/2.8L IS MkII USM and either the two Extenders EF 1.4MkII and MkIII, is in the order of 30%~50% SLOWER than if no Extender is used. The Extenders EF 2.0MkII and MkIII are usually quoted as having a tad more SLOWER AF acquisition, though no tha much slower.
These are (almost always) a bench test using a fixed Subject and then measuring (in "time taken") the AF acquisitions comparatively from several prescribed out of focus distances to difference in AF acquisition distances. The results are then averaged in some way. It is noteworthy that some tests do not even describe the methodology used.
However, whilst I acknowledge that theory, I think that
the salient point is to ask “what does that mean in real life shooting?”
And as a result of asking that question, I consider a more useful, though less scientific approach, is to tally the number of shots lost as a result of AF being too slow.
Note also that one major point being, whilst we accept that the AF with a x1.4 Extender EF will be slower - and let's say it is 50% SLOWER - the critical question is:
"How much IN TIME is that slower?" – and purely from my analyses over several years, using several cameras, it is not that much slower - for my purposes and outputs.
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Another salient point is, (most of) the bench tests which tabulate the relative slowness of AF are intrinsically biased askew of what is, actual and typical real-world shooting.
As an example, let’s take a simple bench which is designed to make an A/B comparison of the time taken to acquire AF using a Lens ALONE and then that Lens with Extender EF added. Let’s assume beginning at four OoF distances and then measuring the time for AF to acquire the Subject set at four different Focus Distances. The two resultant grids would appear like this:
http://gallery.photo.net/photo/18195319-lg.jpg
In this grid, we assumed that 0 (zero) is time is taken to acquire AF to any distance from which the lens is set as a starting point. This is not so: but is is a very short time and I used "0" for simplicity.
The grid implies that the greater the actual acquisition distance from the staring point the longer the time taken to acquire (all other factors consistent). This is actually so.
We would make two grids of results one with the Extender and Lens and one with the Lens 'naked' and then quantify the DIFFERENCE of the time taken to Acquire AF - and as mentioned most test results then describe the difference as a "percentage slower" figure. And the test summary is such as:
"using the Extender EF the AF acquisition is 50% slower.”
However in most typical real world shooting,
the AF acquisition is from a starting point which is around and about the final “acquired” Focus Distance, and this is so even if the action is moving (quickly), so therefore this type of bench testing will provide biased results which are skewed toward the actual impact of the “Slow AF” appearing in print much more severe that it is in reality and for mostly all practical purposes.
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"Practical" AF Acquisition & 'Slowness', in my experience - details:
All three of those mentioned 70 to 200 "L Series" lenses work very good to excellent with the 1.4 Extenders EF MkII and MkIII. The ultimate and the best combination being the EF 70 to 200F/2.8L IS MkII USM and the 1.4 Extender EF MkIII.
BTW - The worst being either Extender with the EF 70 to 200 F/2.8 L IS USM.
Moreover apropos AF acquisition and taking into account the type of shooting I generally do, that being Field Sports such as Hockey; Rugby; Football (soccer); Cricket and indoor Swimming, my opinion is based upon using Centre Point AF in Daylight and Telecast Floodlight Banks - typically EV = 9~15. The three lenses were used variously over 12 years with: 20D; 30D; 50D; 7D; 7DMkII; 5D; 5DMkII; 1DsMkII; and 1DSMkIII Camera Bodies.
I cannot honestly recount any major trouble with AF Acquisition which lead to shots being lost.
For a shot that is static for example a B&G at an Alter - any of the three 70 to 200/2.8:L's and Extender will eat it.
The main reason why I would choose to use a 70 to 200/2.8L and an Extender, rather than the 400/2.8L is, after considering the Image usage are WEIGHT (about 3lbs vs. 12lbs) and manoeuvrable factors. I use the 70 to 200 + Extender mostly always hand-held, as opposed to using the 400/2.8L on a Monopod and thus the 400/2.8 slows me down, especially running a sideline, or I have to dump the Monopod - and on some Ovals I am not keen to do that.
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Thanks useful information: 
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Originally Posted by ameerat42
the $15 a day 
Am also prepared to change lens.
