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Hi David - I've used both models of the 70-200mm f4 series (non-IS and IS) with a 1.4x extender primarily for astrophotography.
The stars are excellent test subjects to highlight any optical deficiencies in such combinations of glass.
I found the non-IS and extender combo to be the best - little to no CA visible on bright stars, the IS version didn't fair so well with noticeably obvious CA around the brighter stars.
Perhaps there's some variability between lens examples - but that's my experience.
The non-IS + extender would be my choice - light enough as a walk around lens and serious bang for buck!!
I recently decided to sell my extender due to little use on my 70-200 f4L IS and go for a longer FL prime - the 400mm f5.6L (waiting for it to arrive). Chris (hotspur) will tell you how good that lens is for birding I'm sure.
All the best
Doug
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