I do have the same problem when I'm using my 100-300 on my G5.Focus is quick and accurate usually but I have had problems when photographing individual birds sometimes where the focus is a bit out, I've tried various metering modes but haven't yet worked out a solution, you only really notice it when viewing 100% but it's a pain.
It is worst when I can't come closer to the birds and the those are not filling a big area of the frame.
Most of the time the birds are a bit out of focus.
I've tried several settings but was not abble to get a combination where I can get at least 50% of well focused photos os birds.
Does anyone have success with a particular combination of camera settings?