As quoted the 300mm f/2.8L is away by far the best lens at this focal length.
Also there is no other lens that will retain picture quality like this particular lens when using both the 1.4x and the 2x converter.
Just a word of warning!!!!!!!!!!!!!
If you do take up Dreeders offer of the loan of his lens be prepared to spend some money, After using this lens you
WILL want one. There is no substitute.
I can speak from experience as I have this lens along with its bigger brother the 500mm f.4L IS.
If you decide to take a cheaper option then look at the 400mm f/5.6. No IS but not a problem IMO. Around £700 from kerso.
One of my sample shots using the 2x converter making 600mm.