I'm not sure who makes it's but the one I just received from Jerry Dover looks GREAT! I've seen numerous ones from our local kart shop, not made there, that are advertised as thin wall returned due to breakage. The prob seems to be that the welded brace isn't heating/expanding as fast as the pipe itself and it is cracking the exhaust tubes. I've seen 3 do this recently. I believe they were the DrSpeed thin wall brand.
The one Dover sells is bronze tig welded and is supposed to flex some. The thick flange is ported well and I must say, having held both feels much more sturdy built.