Have you tried http://photography-on-the.net/forum/ ?  I am pretty
sure you will find the answer there.  Meanwhile, I have the 300mm with
the 1.4x TC and have found that there is minimal loss of sharpness
with it, if any, depending on how close I am to the subject and how
bright the day is.  I haven`t had either for very long though, so you
probably need advice from people who shoot a much wider range of
subjects than I do (small birds, dragonflies, butterflies).

Kate

I don't know if you saw this.  I just from a trip,
so a little late in posting to the rpd newsgroup thread.
I added a few more notes too.
----

I have the 500 f/4 and 300 f/4 L IS lenses.
When I travel light and leave the 500 at home, I'll take the 300
with 1.4 and 2x converters.  Here is an example with full size
crops done with the 300mm and 1.4x TC.  Taken with a wimberly
sidekick on a carbon fiber (gitzo) 1228 tripod:

http://www.clarkvision.com/galleries/gallery.bird/web/lorikeet.c04.07.2005.JZ3F8
962.b-700.html

I also have penguin images done hand held with the 300 f/4 +2x that with the
IS on are similarly sharp (someday I'll get them added to the website).
The 300 f/4 + 2x autofocuses nicely on the 1D Mark II, but it
is a little slower than with the 1.4x.  Also, I use Kenko Pro 300 TCs.

Here are some more 300 +1.4x TC (with a tripod):
http://www.clarkvision.com/galleries/gallery.hawaii/web/hawaii.wave.c12.11.2004.
JZ3F2615b-600.html

http://www.clarkvision.com/galleries/gallery.hawaii/web/hawaii.wave.c12.11.2004.
JZ3F3106b-600.html

This sunset shows the 300 f.4 L IS has no flare issues, this with a
Hoya supercoated UV filter on:
http://www.clarkvision.com/galleries/gallery.hawaii/web/hawaiian.sunset.c12.10.2
004.JZ3F2540a-600.html

Roger
Photos, digital info at: http://www.clarkvision.com