The theory with the slant head that's supposed to be for "performance" is the spark plug's orientation, angling the plug allows for more of the spark area to face the fuel/air pulse directly - if
properly indexed and
the head is on the motor in the correct direction (plug on intake side).
Thing is, not only are the benefits of plug indexing somewhat controversial in the first place even amongst the pros (if there's not a profound enough difference to prove a result - is there a result?), if you do what baird's suggested - search through those that claim performance increases, you'll note that the head's installation is completely random (tho I've not seen one on sideways yet lol) with no
mention of proper plug indexing...
Which leads me to conclude that the reports of performance increases to be mostly based on the placebo effect and/or an inadvertent increase in compression by repairing an existing head gasket leak/thinner than original gasket (which can ofc be done w/the regular head).
Which leads us back to "if there's not a profound enough difference to prove a result - is there a result?"