Mugen products are proven performance gains. What you need to ask yourself before buying is whether the gain is significant enough over your other options to fork out the money for. In the end that is only something you can answer but youd have to know the difference first. And unless you find a dyno graph comparison on the same set up it would be hard to say. I would run the mugen if i had the money. If i found something else that is very similar in performance for less i would get that. I cant speak about rsx set ups because i have never done the research for parts relating to K series motors. Parts for k series will for the most part behave differently than a similar part for the b series. Ill bet the Mugen outperforms however.
But lets simplify. If hes willing to spend the money then buy the Mugen. If not, just buy the sri.