OM-5 ISO at 4,000
that there's a big difference between hi ISO performance and lo light performance. Try a shot at ISO 4000 lit by just a 60w bulb and the noise will be pretty chunky.
But it is a nice birdy pic and for long lens use in those kind of conditions ISO 4000 is really useful.
True, but I don't know anyone that would use a 300mm f6.8 lens to take birdy pics in the dark...
I used to be really hesitant to use high ISO's for anything, but under certain circumstances as you say it's a very viable lens combo.