Really like the direction you are going on this project. Been wanting to do this myself (mainly for the weight saving advantage), but read somewhere that a certain amount of heat has to be kept in the pipes to maintain exhaust efficiency? Supposedly this is lost when the wall gets thinner, anyone heard of this?

I built an aluminum silencer for my snowmobile before the aftermarket offered them. I modeled it off my motocross bike: used a thin-wall metal pipe, perforated it, made end-caps, and riveted an aluminum outer to it. It didn't last: blew out the rivets so I re-did it and doubled the rivets. It weighed maybe a tenth of what the factory system was, so I achieved the goal.

I guess the moral of the story is that this is why the factory systems are so freakin' heavy: so they last the life of the car.

So, never one to give up, I went to work on my Yamaha 660 Grizzly.

Here's the factory stainless system that weighs a ton:

And, I got a factory exhaust from a Yamaha YZ250F and broke out the MIG and adapted it to the Grizzly. Easily lost 12 maybe even 15 pounds, the YZ system is feather-light and does a reasonable job of keeping the sound down. Same principle applied to a car should be an easy 20 pound weight loss...........

yeah, keeping heat in the pipes. I've also heard some thing to that effect as well. I just don't know how true it is. I mean, open headers do not keep heat any farther than the collectors length and some car run hard like that and some run great with complete exhaust systems. The aluminum tubes that I intend on using now are thicker than the stainless 304 tubing that I have been using for the sake of not distorting out of shape and keeping the sound to a minimum. I think that exhaust tube/muffler thickness experimental sessions at the factory have to do with sound retention. I could be wrong. If anyone on here has another opinion, then feel free to cut up my theory. LOL.

I remember running Straightline Performance Titanuim bullets that were super light and super thin and were quite noisy for the street, then I I purchased the Dynomax Ultra flows and the noise went down substantially.

So as I understand it, our motors are approx 33% efficient, ie 1/3 mechanical energy to the drive train, 1/3 waste heat ie radiator, radiant heat, etc, and 1/3 acoustic energy out the exhaust. So when a muffler absorbs/converts this acoustic energy, it is converted into heat, and therefore mufflers get hotter the more effective they are in reducing acoustic output.

Seems maybe alum in this application is only for a poorly executed muffler design

Well, since I'm in it with materials and such, I will go ahead with my Mad Scientist experiment and see what it sounds like and performs.
I drive my car as a weekend warrior only,(NOT A DAILY DRIVER) and I also try and drive to the local tracks 50-70 miles one way, so I will see once I have all the gremlins ironed out on how both exhaust systems play out in real world testing in similar weather as well as final weight numbers. If it works out, then I will of course be transparent to what I have achieved.

Another reality of Aluminum that gets exposed to many heat and cold cycles would be a tendency to crack. Well, I think that depends on what type of mounting is considered.
If the front of the exhaust system slips completely over the ends of the collectors via an expansion male/female joint and the rear is mounted with traditional hangers that incorporate rubber straps between the mount and U-strap in order for it to flex with irregularities of roads and forces, then I don't see how it can crack. There's no stress.
Only if it were mounted solidly to the under chassis would I see a problem with most alloys including steel. Look at the case with cracking in Chrome-moly tubing on some occasions.

Had my friend run weld beads tonight and tie all the primary pipes together, now the task of Muffler building and welding to finish it all off. I'll post flicks when it's all on deck and ready for its closeup.

I gather that Ceramic fiber is the general term for many products including Mineral Wool Right?

Thay sell the wool and ceramic cloth/packing.The ceramic is about 20$ that should do 1 muffler.

Mirror, Mirror on the wall, who's the lightest system of them all?
Here's my Aluminum muffler Hangers compared to their steel counterparts. Saved a whopping 1-1/2 Lbs.

3" round tube Muffler Hangers.
Next I will show you'll the Aluminum mufflers themselves when completed. Should save a good 20 Lbs alone.