The new sigma 50-150 f 2.8, what improvements

Have been made to this new version to justify the 200+ dollar price increase?

Specifications appear to be identical. Here's Sigma's marketing blabbage: "The version II of this high-quality lens has been optimized with an even better optical design, which provides optimum lens performance providing excellent correction for flare, ghosting and all types of aberrations." So, maybe there were some issues with the original that Sigma fixed and is marketing as "improvements".

I wonder if an owner were to send her lens, under warranty, back to Sigma complaining about excessive flare or aberrations, what would they get.

The price difference on B&H is $140 - 679 vs 819 (for the Sigma version).