Except for possible AF performance advantages (unknown but probable because a TC forces a reduction in AF speed, at least on EF lenses), the main advantage I see is that these lenses with an internal TC also take an external TC with no loss of infinity focus.Pricing is bizarre. RF600 + 2x TC is $13600, instead of $20k for the same lens without the sbility to detach the TC. The reviews will be looking carefully at whether the 1200 has meaningful advnatages in performance, weight, and size for the eye-watering 50% price premium. (RF800 less so but still painful).