Hey Peter I would agree with your friends' advice about getting the 400 over the 300. I had the 300 for awhile, and I used it with a TC 90% of the time, and i thought it worked well. Then i sold it and bought the 400. The AF speed and IQ blew me away compared to the 300 w/ TC. So if you use your TC often i would just get a 400.
Im also waiting on more reviews on the Tamron as Focal length is almost as important as IQ and i would love to have more reach!
Its gonna take a lot to pry that 400 5.6 from my hands though!
My problem with the 400 5.6 is me!! I am getting old and can't hand hold it anymore. I love this lens I think for the money it's one of the best