According to SAR the lens will be announced next week:
Compare to Sony GM,At the center of resolution, it is good as Sony GM.
At the corner of resolution,Sony GM is better.
AF speed is slower than Sony GM.
Sigma Art is much lighter and cheaper.
Due to issue of 28-70 Art, Announcement is delayed.
Sigma will start to sell this new lens July or June in this year.
And it will be announced on April 27.