The Hemi pistons are domed (but I don't know how much).  Usually in automotive engines, the only time a dished piston is used is to lower compression.  On a Hemi engine, the spark plug is located in the middle of the cylinder head-directly over the middle of the piston.  The combonation of the domed pistones and the Hemi heads consentrate the heat to the middle of the piston where the burn produces more power.  Overall, this piston and head design is more efficent than the standard design.

As fas Dodge reintroducing the "Hemi" engine, I think its more of a sales ploy than anything else.  The old Hemi's were very popular performance engines and by reintroducing an engine with "Hemi" components (especially since it is a 5.7L small block-just like the legendare 5.7L GM small block which has recently been discontinued), it's a great marketing move.