How well does the 135L take extenders for those of you who have it?

It appears the 135L is one of those "what did you use to take that picture" type of lenses with top of line performance at 135. I was wondering how well it performs as a 189 f/2.8 and 270 f/4.

I understand reading other places that the 135L is still quite good with the 1.4 extender without that much reduction in focus speed. How about at 270mm? Is the autofocus still useable or is it painfully slow?

If you stop it down to f/5.6 or f/8 will it still smoke consumer zooms such as 75-300 IS and 55-250 IS at the far telephoto end? The digital picture comparison tool seems to imply that it will:

http://www.the-digital-picture.com/R...mp=2&APIComp=4

I'm using very basic lenses right now on my 60D. I have a 18-135 that I generally like for landscape use and use a 55-250 IS that is hit and miss. Some shots, like the cardinal picture I posted in the bird thread, I really like but other times it lets me down especially with its autofocus.

I'm strictly a hobbyist and am looking eventually to possibly upgrade and get a better general use telephoto lens. I rented the 70-300L and loved the image quality, but it is quite large, heavy, and not that fast. I was wondering if going prime was another possible good alternative.

Thanks
Dave