Maybe a little, but putting it directly in the intake will help out more.
S197 does have an EGR valve. It's located on the inside of driver's side fender under the hood. The little black plastic piece.
I never said that injecting it into the intake may not help more than pouring it into the gas tank.
I only said that pouring it into the gas tank does more than only lubricating and cleaning the fuel injectors.
FWIW, 1 can of it eliminated a pinging problem on my 05 GT.
According to the following spec, which was taken from a Ford pdf file, the S197 GT does not have an EGR valve(at least on the 05 model):
Vehicle Line: Mustang Coupe
Model Year: 2005
Engine Description: 4.6L
VEHICLE EMISSION CONTROL
Exhaust Emission Control: Exhaust Gas Recirculation
Type(controlled flow, open orifice, other): N/A
Exhaust Source: N/A
Point of exhaust injection(spacer, carburetor, manifold, other): N/A