External CSS seems to work fine, and is useful for a responsive site. So, for example you could add a class to the INS. Then you could set this class to width:320px; height:50px for your mobile layout, and width:728px; height:90px for your desktop layout. Just make sure you remove the inline width and height settings.
It seems to work fine.
Reporting is not so easy as its all coming out of a single ad unit. It would be useful to set different channels for each of the different CSS size settings.
There is no noticeable difference in performance. The new adsbygoogle.js code still makes the same calls that the old show-ads.js did. So you still get that latency as all the ads show up (typically well after the page is rendered).
However I guess the thinking is that because the new script is no longer blocking, if there was a slow adsense server or it was down, then it won't stop your page from rendering anymore.