as above, shortening runners on rb20's not a good idea
i have put on plazaman plenum (same as roy in build thread mentioned above) my car is rb22 specifically tuned for high rppm for drift... i got 200 rpm more response and a decent more power held for longer at higher rpm... and a neater engine bay
but.... i have everything already and it was the last thing i did and wouldnt say it dramatically increased performance... unless you want a nicer engine bay or really really want to do it... leave it as is