I have both Panasonic lenses and the 100-400 still doesn't really do it for the tiny birds. The difference isn't as much as you'd imagine. It's better but not by a crazy amount. I think you need the Olympus 300 pro with the 2x teleconverter if you really want reach.
Your copy of the Oly 45-150mm is faulty and that onesided softness is not a typical characteristic of this lens! I´ve used two of these and checked multiple reviews and the lens should perform much better on the lower zoom range. Maybe it is decentered or something like that.
That could be true, KapeaAurinko! That is a good point. Another website that does resolution testing with Imatest did find that the corners are significantly weaker below 70mm, so there should be SOME corner weakness though.
