This is a difficult question and I think it depends on the copy of the lens and the converter version. Also good lens / body AF adjustment is important.
Personally I think the "new" 500 IS II and 600 IS II are a only "little" better IQ wise (sharper / contrast) to the "old" 500 IS and 600 IS. AF performance is not so much a difference and IS is of course improved. If you are happy with the old lens I do not know if the upgrade from the old IS to the IS II makes sense...
When adding a 1.4x TK III, the new IS II lenses are superior to the older IS lenses with added 1.4 TK. Also AF works faster.
Adding a 2x TK III to the new IS II lenses and tracking subjects only is possilbe with the 5D III and 1DX. For expample I can still track birds in flight easly with the 600 IS II and TK 2x III - even with not optimal light and a good keeper rate.
When you cannot get close enought to the subject - then I think the IS II lenses with the 2x IIIer Converters make sense. I personally find the IQ of the 600 IS II with the 2x III Converter far better than the 7D with the 500 IS and 1.4 III Converter. It is hard to believe but I actually think of selling my 7D now - but maybe hold it just for the CPS Membership
A friend of mine uses the 500 IS with 1.4x II Converter and also switched from 7D to 5D III. Since that day his 7D is only beeing used as a backup. Now he also starts to use the 2x III Converter with the 5D III after the firmware update and is very happy.
Lets face the reality:
7D is an outstanding camera but IQ struggles with higher ISO.
500 IS lens is one of the best wildlife lenses available. You still get very good image quality with 1.4x and decent with 2x.
5D III is also an outstanding camera and if you do not live from wildlife photography there is no need for "more".
The new 500 and 600 IS II lenses are maybe the "best" available wildlife lenses available. Adding 1.4x almost shows no difference than without and adding 2x the image quality is still superb.
The 7D II is not beeing released soon and who knows which features it will provide. I personally would actually go ALWAYS with the 5D III which gives you also everything you need beside wildlife.