WE sell both and my responce remains the same..
On the ole flatheads I could show you on paper gains in performance
On all the OHV engines we have not seen documented HP gains with them..
BUT they do cut down on radiant heat,, won't rust and if its the good Ceramic Silver Coat then it shiines like chrome and is down right pretty..
Even though I make this statement we still sell 3-4 of them out of 10 headers sold..