For example having a big valve head with all the port work done etc,id expect gains adding this exhaust manifold.The difference is,port 2 and 3 are no longer joined with the same exhaust exit as the ford item does.Spose to help better exhaust flow.
Think ill get in contact with the guys and see what they say.Still tempted to buy it myself.If it don't work,then ill stick the old one back on.Try and error.