If you go the extender route, here are a few shots I found that I had on flickr using the 1.4x II. They are not the best shots but hopefully will give you some idea. I can probably dig up some better ones if need be.

This one is with the 200mm f/2.8L II @ f/4...

IMG_6025_DxO by Denise Trocio ( www.dtrociophotography.com), on Flickr

This one is with the 135L @f/2.8 ...

IMG_9254 by Denise Trocio ( www.dtrociophotography.com), on Flickr

100-400mm (manually focused) @ f/8 ...

Male Cardinal by Denise Trocio ( www.dtrociophotography.com), on Flickr

70-200mm f/4 IS ...

IMG_0015 by Denise Trocio ( www.dtrociophotography.com), on Flickr

I have yet to use an extender on my Sigma 50-150mm OS lens but am very curious as to what the results would be like.

Good luck with your decision and be sure to let us know what you decide on and post some photos taken with it.

Denise