Thanks Gronk. Do you shoot smaller birds in flight? How does the speed of focus on the tammy compare with the Canon 400 prime? That's now my only major concern about trading (apart from weight).
To date I have not had a chance at doing any small bird BIF work, but over all the focus speed between the two lenses are about the same, although I have yet to try the lens out on a fast moving subject.
To date I am more than happy with the lens and I can't see there being any problem when I come to try my hand at BIF shots.