I'm finding the 100-300 very, very slow on my E-P1 - even subjects like this can be quite challenging for it, whereas the 40-150 M.Zuiko is quite noticeably faster.
Do others have better experience with the 100-300 on more recent bodies? Is it any faster, or just as slow?
Nice dog shot!
The AF technology in M43s has come a LONG way since the EP1. The 100-300 is a reasonably fast focuser, but the EP1 is pretty pokey compared to current M43 bodies. Also, there has been at least one firmware update for the lens, which might help, if you haven't done that yet.
I'd see if you could try it out on a newer body (current generation, if possible, of either Oly or Panasonics) to compare to what you are using now. You may be surprised at the difference.