IMO header wrap will be most effective on normally aspirated engines when the exhaust system is perfectly sized or a touch undersized. Most folks can't help themselves when getting a performance exhaust or header though and have it too large, velocity is already sub-optimal, and wrapping the header will have marginal gains. On a correctly sized exhaust, which may include many stock exhausts, I think it will be more noticeable. However stock exhaust systems can be under or oversized too. Wrapping might help an undersized one on the low end quite a bit, but it will do practically nothing for an oversized one until it's bouncing off the rev limiter.
When the exhaust system is tuned perfectly, the momentum of the exhaust gases provides a small amount of suction just before exhaust valve closing, that helps extraction, may provide a small amount of positive effort to the piston (depends on timing) and with valve overlap, can kick start the movement of the intake charge, which is sitting static just as the intake valve opens. Result less parasitic loss.
So, what you're hoping for with wrapping is to keep velocity high in the header and downpipe, not allowing flow stalls due to cooling.
The way I'm thinking right now, I believe that short of a custom header, the piping sizes are about right on Marvin, but his exhaust ports need a little restriction... on Wile-E... the exhaust ports and cast header are cavernous, twice the size they need to be, but the rest of the system is a touch undersized, it could do with being 2" all the way back, but it's only 1 7/8 or 1 3/4. Might need 2 1/4. I will have to see how much effect high compression modifications and porting makes. I'd like to get Wile-E a stock '90 or earlier escort GT header, needs some modification though. Anyway, plan to wrap things up for Marvin when the "new" motor goes in, and assess Wile-E's needs when I see how the mods run.
Being a cheapskate, I'm probably just gonna buy generic woven glass tape, not the same stuff priced up as header wrap.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
If you can imagine cold air on the intake side improving power, then you should be able to imagine the same thing for hot air on the exhaust side. In theory, you want the exhaust to stay hot all the way out. The same way you want a turbo to get hot. But in practice it's highly unlikely that it's enough to make any noticeable difference to either power or fuel economy.
I had a large section of my exhaust wrapped on my previous Civic in order to try to reduce the noise. Didn't work for that either.