I think the A1 will provide a great alternative to using a teleconverter, in my case with the 200-600mm. I have the 1.4x which I use on occasion and I have wondered about getting the 2x. APS-C mode will replace the 1.4x with the advantage that the AF is still using f/6.3 and so will hopefully keep the speed up. Also, it can be engaged at the flick of a switch. And in the rare case of wanting more reach, APS-C mode + the 1.4x is probably an excellent alternative to the 2x. I wouldn't expect the 2x with APS-C mode to be even remorely useful or the 2x in full-frame mode to be better than the 1.4x in APS-C mode.
So the cost of a 2x teleconverter can be deducted from the purchase cost, in my head anyway.