I use to run headers on everything I owned, but I am not sure it was helping that much. I am able to get the head flanges to always seal by retightening the bolts frequently during the initial run in, the gaskets compress and will burn thru if u don't. Sometimes u have to use two collector gaskets to take up the gaps in the flanges, the rings are too thin to compress the joint (just bends the metal).
I have also had mixture problems due to the pressure waves in the primary pipes and low restriction mufflers.
It may be worth 5-15 HP at some points in the RPM range on a mild motor, IF they don't screw up the jetting.
And ditto on ceramic coatings, 35 years and I have yet to find paint that won't start to rust through within 6 months. Hardware store Barbeque Black seems to be the best IMO, if you sandblast the old stuff off first.