No thanks @ throwing away the bishop pair so early. 

Paulby: Development over all... the c1 Bishop has to get out and furthermore, 8.d4 helps open files to the centralized king.

I don't understand why 8 d4 allowing the cature of the e pawn is a standard move looks weak to my inexperienced eyes?

The point is pretty strait forward... in most any line Black chooses, the best option for King safety is to remain in the center; with foresight, White ascertains the best option is to prematurely open lines to the centralized king. Therefore, this e-pawn sacrifice can be thought of much as g/h pawn sacs in the Sicilian Dragon - Yugoslav Variation that are played in order to open attacking files. Plus, keep in mind that the d or c file will at least become semi-open if not fully open.

a change-up used against sicilian players?

What's the point  to this opening?