In built TC performance sounds pretty good. This effectively gives you a zoom with a focal length of 200-560mm AND it can be used with a 2x TC also.
Looking forward to it ... was planning to get the 600mm by the end of this year but this lens gives very nice options (except for the 1 stop sacrificed for the TC beyond 400mm). Hope the production models are as excellent as this review makes it out to be.
1DX, 5D3, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L II, 50L, 85L II, 135L, 17TSE, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 1.4x III, 2x III, 600RT x 4
When it comes to gear, it is better to vote with your wallet. Bitching on online forums like a lunatic doesn't get you anywhere!