I'm not sure what you're asking?
If ya mean adding a pipe from the exhaust port on the cylinder to the Spooky expansion chamber (making yer own headpipe) you'll get better performance than no headpipe at all (so long as it's about 12") & you can even use the kit headpipe, flange n'all if ya just cut off the muffler and weld the expansion in it's place...
As for better mileage, just about all performance modifications increase efficiency which means you can
get better mileage - the trick of it is to try not to hold it at WOT alla time... which I must admit I just can't resist
TBH, tho these lil 2 smokers do get good mileage - that's not their strongest attribute (which would be power to weight BTW), if ya want the best
fuel efficiency - that's the 4 stroker's game.