Well... I drove it around 40 miles, on a 60 degree day, and the tops of the sills were VERY hot. No heat (that I could tell) inside the car, but the sills themselves were as hot as my 97 gets. I don't think any hotter, but you will burn your leg if you are wearing shorts. I think the difference is the sill area on top is a lot "thicker" than the old sills, so your leg has more chance of "hitting" the sill, so more chance of burns...
I checked around the sills, and I couldn;t see any crazy cooling ducts or anything, so I'm thinking they just threw some header wrap, or other coating in there.