I've found that when the Panasonic seems to be struggling to focus on the subject wanted, if I manually focus somewhere near enough to 'give it a hint' so to speak, then it locks on quite well. It's one of those things where you have to keep trying stuff out to see how it works and eventually you get to use it instinctively - though, to be honest, I haven't had the time that I'd like to use the whole kit as much as I'd like recently.
DXO rates the Olympus 75-300mm lens as a bit better than the Panasonic 100-300mm, but it doesn't have the close-focus at the long end of the zoom range to give a more comfortable working distance for nervous butterflies, dragonflies etc. The Olympus, of course is a bit slower (F6.7) at the 300mm setting, though it's a bit lighter in weight but doesn't include a lenshood (or pouch case) in the package.
A couple of shots with the Panasonic at 300mm on my E-M5 below. The Banded Demoiselle damselfly is one I've already put in my 'Birdforum' gallery and the Iberian Marbled White butterfly is one I placed in the 'Surfbirds' insect gallery as that species hadn't been featured there before:
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