Tony, FWIW, I moved from Canon 7D2 albeit with just 400mm f5.6 (& sometimes 1.4 TC) to Olympus nearly 3 years ago & once I got familiar with the EM-1.2 which I originally bought wouldn't think I'd ever go back (getting used to mirrorless after DSLR was biggest difficulty & took some time to get used to).
I started out with the 300mm and the 1.4 TC just because I found them at an almost unbelievably good price for sale with the EM-1 from someone who couldn't get on with mirrorless! I've since got the OM-1 (April this year) and in-between also the 100-400 zoom & the 2x TC.
In practice I almost exclusively use the 300mm, with the 1.4 TC more or less permanently attached. The 300mm and OM-1 give you full sync-IS which I think is somewhere around 7 stops whereas there is no sync-IS using the 100-400 zoom so nominally around 5 stops based on IBIS. In practice I've found that IS with both lenses with and without TC's to be exceptional although I rarely use a TC with the zoom as it does slow down autofocus and anyway light in the UK doesn't often encourage it! Personally I think the 2x TC is pushing it in most cases for the zoom though on the 300 it is excellent and I readily use it.
I've found the OM-1 to be a huge step forward for birding - both static and even more so for BIF's. The bird detect option works well & I have it set on for most birding situations, the exception being where the target is partially obscured and I need to try to get a single point on the target to try and ensure focus is on the bird (there have been occasions where the auto focus is thrown by the vegetation if I use the C2 options below):
C1 - for difficult focussing situations, ie. target partially obscured = S-AF, single point focus, bird detect OFF
C2 - most other non-flying targets = C-AF, small block focus points, bird detect ON
C3 - BIF's = C-AF, all focus points, bird detect ON
If you're interested in seeing some results I've just loaded some from our visit to Costa Rica this spring onto flickr - that includes quite a lots of BIF's and also plenty of forest-dwellers where light was minimal (at worst, down to 1/6th hand held at 840mm effective length for Scaled Antpitta). Not sure how they'll hold up against your own photos from CR as I have no claims to be any more than a birder taking some record shots!
Highlights of our birding trip to Costa Rica: just the two of us and a brilliant local birder/guide/driver who sorted all the logistics and kept us birding from dawn to dusk (or later) - which is what we wanted. Selection from the 15k+ photos of 311 species out of a total of 534 species seen!
flic.kr
