I wouldn't worry overmuch. I've skied up there about a dozen times over the years. I can only recall one time that it was really quite cold at the summit. -14-18 if I remember correctly. Most of the time it is quite nice. You can check seasonal averages for the town, at 800m altitude. But then compare the immediate temp in town to summit temp to get an idea of diffs. If it is really windy the lifts will get shut down anyway.
( It occurs to me that, living only 2 hours away, I tend to stay away when the weather is known to be bad for a day, so my experience may be more rosy than someone who flies in. Still, overall the weather is not very harsh).
(right now 12C difference Whistler summit to town)
can't seem to bring up a decent history from wunderground, keeps on putting me at Vancouver airport:
Pemberton is near to Whistler, although it has a bit of a microclimate. Good average temp graph tho:
Grab waterproof hardshells along with underlayers - maybe an extra set should you need to double-layer, which I doubt. Std ski gear, really. The town is very-well equipped with outfitters should you need to upgrade your cold weather gear. At a considerable price, of course, Whistler being Whistler ;-)