No you are right that he will not NEED a larger plenum, but being a common fact that forced induction motors should have approx 75% engine capacity in the plenum, tapered off to avoid barrelling at the ends (note: TB is located in centre of plenum..very messy air flow) (resonance plays no part with forced induction), the manifold is currently OFF the car, and theres been oh so much custom work, it would be a wise step, especially since the single turbo setup will mean a 'top endish' dyno curve. Its just something I would do if I were going that far. And yes a larger diameter exhaust is always positive with forced induction as %0 backpressure is as efficeint as the turbo can get.