Also with Mugen, unless you have an aftermarket front bar, it's hard to say that you would get more gains than you would with a GruppeM purely based on the design of where the air is being sucked from. The OEM front bar restricts the flow of air for Mugen whereas GruppeM sucks on demand from the windscreen Just reinforces what Temp says and this is clearly shown on data taken whilst tuning.

Btw if you're worried that you need to cut bits and pieces to fit the GruppeM, this is incorrect, GruppeM provides you with the vital parts so this doesn't have to be done. Thought I might point it out cos I've noticed some people get the wrong perception