Any situation with substansal gains. If ur getting mani's and dp's deff go with mike smith( or if ash's new design comes out in time may want to check those out) and sz 3". With the dp's the deciding factor would be 4 bolt or 5 bolt. So if you are upgrading and what you upgrade to turbo wise must come first. I can't speak for 400rwhp but if u only want 400hp at the crank intercoolers manis and dp's should put you well over that.
"torque gets you off the line...& hp gets you over the line." - TTKeeper