As a PPV owner, with a long history of developing and implementing brake upgrades on the B-body (94-96 Impala SS), the key thing to understand is that fitting as much rotor mass as possible inside the specified wheel is the first consideration, not whether the caliper has 1, 2, or 4 (or more) pistons. More pistons, by themselves, does not always equate to greater total braking force.
The PPV & Camaro LS/LT caliper is a single (floating) piston design--simple, cheap, and reliable, which is what a fleet operation (like PPV's) is all about. No bling factor--call it effective and unsophisticated....
The PPV's larger 345mm front rotor diameter, combined with a more aggressive pad friction compound, provides better thermal "management" and develops superior braking torque compared to the 321mm G8 rotor and twin-piston caliper package. The pads are quite dusty, however, and there can be some degree of brake squeal under certain conditions.
The single-piston caliper also allows this larger rotor to be packaged inside the PPV's 18" steel "cop" ventilated HD wheel with no clearance problems.
Bottom line - I would consider it an upgrade on a G8 (GT) because of the increase in rotor diameter/mass, in spite of the change to a "lowly" single-piston caliper. Your other option there is to look at the Cadillac CTS (not V) with J55 brakes--the same 345mm rotor is used (I think) with a twin-piston caliper (and different pads), but I'm not sure whether that setup can mount to a G8 front upright, or if it would clear an 18" G8 GT wheel.