I haven't tested them in GT5 against each other but in real life the Spec V has (among other things)
Increase boost button (boost scramble) giving more mid range torque and a retuned exhaust giving roughly 5 every peak HP.
Light weight Rays forged wheels
Carbon Ceramic brakes
Revised suspension geometry and materials
Removal of rear seats
and many more small things I can't remember off the top of my head.