Jeff, I shot with a Nikor 300 2.8 for a long time on my Fuji S1 & S2 and with a 2X got nice results most of the time. I do wildlife for our Endangered Species Unit here in New York with our eagles and have purchased a Sigma 800 5.6 and was so impressed with it I went and got their new 120 to 300 2.8. I have not been dissapointed with either and the 120 to 300 weights in at about 5.7 Lbs. so I can walk with it and a light tripod or monopod and a 2X and am covered up to 600mm at 5.6. With the 800 I do have to use a heavy tripod and I use a 3 way Pan head by Bogen. I had to use this with my 300 Nikor also. I think you will need to follow the advise of Bob, seems he is right on with what he says. Wild life requires a lens a little longer than 300mm and a good tripod and a strong back.
Most of the shots I get are over 300 feet. You cannot approach the eagle any closer without disturbing so alas my 800mm. Am not sorry.
www.loubuscher.com