Just ordered the Panny 100-300mm for birding
I use this lens on the OMD too. I suggest you set the anti-shock release option (mine is set at 1/8 sec although to be fair I haven't really tested the options properly.) Before I did this I found it almost impossible to get anything remotely resembling a sharp HH shot @ approaching 300mm even when well braced (using IBIS rather than the Panasonic in-lens stabilisation).
Even so I find that shutter speeds have to be 250 or higher at these kind of FLs if you want anything reasonably sharp - which is not exactly surprising at an EFOV of 600mm... One thing worth mentioning is that this lens is, at very least a nice 200 F4. I like to shoot RAW, especially with these MFT lenses, LR4 cleans up the ca that so many of them exhibit.
That said the combo is light and convenient and I don't expect too much other than a record of the birds I see. It's certainly nowhere nearly as good as the D700/Sigma 150-500 system I was using previously even though the Sigma lens is frequently sniffed at by the "stellar" brigade. I wish I hadn't sold it: OTOH I hardly ever carried it speculatively whereas the OMD/100-300 is almost always with me.
Good luck with the lens..