It's all about multi powertrain options. Ford is KING of this. Give us a boosted 4 and a beastly V8! Cost can still be kept low if they borrow the previously mentioned power plants from the infiniti/nissan line-up.
Cheaper car for those who want it. BEAST MODE car for the others
420hp in a Z would be FAST and fun... but still shouldn't detract from GTR owneres. No way would you get a 0-60 in 2.9 with a V8 RWD Z.