You will more than likely gain on the top end, I did a comparison against the kiddracing on the same steup and up top the difference was like 10whp, midrange was just 1whp shy with the 4-1, even trade!
as i recalled.... i went to the stealership and ask if the mugen si come in RR... so they said NO.... and then i said ... can i get these part for the reg si (the aero kit) they said No... so i was like damn these guy are as$hole....so i was like WTF... i went to another dealership and saw a brand new RR FA5 on the truck... and i just got it.. and started modding the exterior first and now engine werk.... i updated my build thread(mod list)...