so the 70 - 200 F/2.8L IS II becomes a 196 - 560?
While F/8 is not ideal, still might add in some use. The TCs already slow the AF down as it is, so manual focus is not as much a big deal.
Not sure the price... Probably somewhere in the $600 - $800 range, but if it works with the 70-200 like the 2x does, I could see there being interest