The E-M1 should perform well with this lens. You won't find many with direct experience this lens is not that common.
Personally if I had the $$ I would tend to buy the 50-200 and the 300 f2.8. The 300 is generally considered sharper than the 90-250. There is a great image thread on here with the 300 2.8 I just can't find it right now...
You might ask ak300 for specifics - he seems to be getting good results with that combination. Considering how expensive and rare the lens is though, you would be well-served if you could rent/borrow/try before you buy. It's certainly a great lens, but it is not without limitations.
The 150mm f2 and the 300mm f2.8 are probably the absolute best lenses Olympus has made. I read somewhere that the 300mm was only made to order, but I'm nor sure how true that is, considering that you could buy them off-the-shelf.