I've been lurking on the board gleaning insights for my summertime kit and figured I'd as might as well share...
For me, I generally ditch the windshirt AND baselayer in favor of the Rab Boreas. I find that it wicks, dries fast, adds a little warmth, and gives adequate wind protection in most conditions.
That being said, I don't ever head above treeline without a hardshell with pit zips (currently the Arc Alpha SL). If the wind really kicks up I open up the pit zips and am usually pretty comfortable.