need opinions/experience with Sram black cranks, I've read that the outer chainring is real flexy, when sprinting. Any truth to that? Thanks

IMO no. I run this group with the HG SISL crank.I have no flex here. So what kind of legs can flex a Red Black or a 2013 Red crank which are two of the bulkiest crank designs and look more to be designed for a MTB group?
Further coming from a DA 7900 equipped bike I have experienced that the 'low shifting record' of the Sram Red FD is a myth for me though the Cdale HG cranks on my current bike.

1st gen Sram Red big chainring were flexy. Several years ago, way before first Red Black...

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Roadbike
Kinesis RaceLight T2

Mountainbike
Van Nicholas Zion Ti