Np, Dennis--even if I'd missed it, it would be good to see a market price...
Eric, yes, as you know, I have the 300 + 1.4x combo... I've never met a 2x TC that I've liked, although I've not (yet) tried the Rollei 2x.
Cropping--what's that?? I just looked it up--you mean
throwing away pixels??! Realistically, I think 420mm f/5.6's about as good as it will get with this setup...
A Hasselblad Tele-ApoTessar was kind enough to post full-sized sample images to this question over at getdpi, for anyone curious about performance. Unfortunately CA was 'fixed' in Phocus (images were shot on a CFV-39)