[Ginola]

You could always have a go at porting (gasket matching) your own, make sure you use safety goggles or you could lose an eye!, Use an air powered die grinder, with a tungsten carbide bur tool and then smooth out with some fine grinding bits.
But in all honesty it won't make much difference, not a noticeable difference at least unless you port match the heads themselves, and that is a ball ache, The up pipe however does make a noticeable difference to spool.