According to this commit: https://github.com/away3d/away3d-core-fp11/commit/31abe8b34cc24c2a514b24f0cbbcddc388479929 Away3D now has ATF support.

ATF is Adobe’s “compressed Texture Format”.

So, what are the advantages of using ATF over BitmapData? Is it worth it for me to convert my png’s to ATF?

Yes the first implementation has been made.
You should find lots of answers in these 2 articles.(and probably more will popup in comming days)
But short answer to your question: if your project is demanding on your target platform, it might be worth a try.

http://www.bytearray.org/?p=4472

http://stackoverflow.com/questions/1686525/pvrtc-compression-increasing-the-file-sizes-of-png

Is it possible to compress a bitmapdata object and send it to the gpu memory?

- are there any possible performance gains to be expected in this scenario, taking into account mobile devices with limited gpu memory upload bandwidth?