Just a quick update.
I received the Kenko 2x DG MC7 tele converter today, and this doesn't have the contacts to power the HSM in the 70-200 so doesn't work with this lens unfortunately.
It doesn't even work in manual mode, and causes strange things to happen with the menu and program select screens, just like the Sigma APO 2x did.
So, until the revised Sigma converter is released, the only options would appear to be the Kenko 1.5x DG converter, or the aforementioned Tamron 1.4x Pz.
I took a few in flight bird shots yesterday with the 70-200 and Kenko 1.5x converter yesterday, and I was pleased with how they worked out.