Any BMW is not a more focused driver's car, so you can rule out any of those. Likewise any Merc. Both make great cars, but they're not more focused than a Zed.
If you want more focused, but without going silly (like a Caterham, for instance) then either you get an Elise variant or maybe a Megane R26.R, or you strip all the weight out of the Zed and modify the stopping, handling and power parts, in that order.
I don't think an S2K is a much more focused car to make it worthwhile changing.