I have made some.
one for a 32sx which worked out great.
the gain was about 1300 rpm's on the top and a much faster throttle responce. Here is a photo of it. I had painted it with primer to hide the design of the can but I have since removed the primer for a more natural look.
I mounted the exhaust with a full length header and then worked it down 1/2 inch at a time. each 1/2 inch i tested different props untill I found what I wanted/needed.
the best part about it is they are cheap.
when you get what you want out of a home made exhaust, it's just as rewarding as building a plane especially when not to many other guys at your field make them so they all come over to look..