As for "proof" I can only go by what I've been told by folks in the GO, and by personal experience.
I have personally flown with (at least) 5 or 6 FOs that were former FAT (pre acquisition). I'm betting that I haven't flown with every guy that came over from AT before the acquisition, but even if I have, that's still 5 or 6 more than left SW to go to AT.
None of them painted nearly as rosy a picture of day to day life at AT as the usual FI suspects do and were much happier here, well, at least until the acquisition announcement. Most of the FFAT guys I've flown with since would've all done better, seniority wise, had they stayed at AT.
It's amazing how much better AT was....... after the acquisition announcement............