[Brightcolours]

I've done it in the past, but in my case (Nikon D100 + AFS 300 f/4) the resulting f/8 max. aperture was barely enough for the AF to work - in other words, no chances of getting BIF. One of the points of thxbb12 is that with f/4 it's reasonable to expect a very good AF speed.

Yep, of course you get an AF advantage. And an IS advantage on your case. And probably a sharpness advantage. But talking about lens equivalency is not talking about how this Olympus lens would be worse (it is most probably a very fine lens and will not be "worse" than equivalents, in many cases actually better). Yet whenever one mentions what the lens is equivalent to (in focal length and f-value) in FF 135 format terms, there are always people who take it that way. There probably is a term for that.

As I asked for some days ago   knowing the size and weight of the new lens is fundamental to place this discussion in context.

Not really. It still is equivalent to 600mm f8 on 135 format, even if it is small and light. Also if it is big and heavy. Lenses being equivalent on different formats in focal length and f-value terms says nothing about the particular lenses' advantages and disadvantages. It is not about "better" or "worse", just about similar properties concerning FoV and DoF. And when one for whatever reason feels a need to have similar exposure times, one just has to dial in an equivalent ISO setting.