To address some questions, no, a Sprinter will not outrun a CG. But ive driven a CG classic as well and it isn't blazingly fast, it's just a bit faster than my bike. And i felt that a GS felt faster through out all the gears.
But once again im leaning towards Piaggio for some simple reasons:
Nice new handle bar
Big tyre at the rear with a suitable rim
3 of these things are absent in the GS150, which costs more than the Piaggio in the first place.