I am getting mine coated on the inside with black ceramic, it will stop a lot of heat from transferring to the aluminium. The reason I am not coating the outside is so the heat that does transfer can easily escape to the atmosphere.
Make sure you have plenty of bungs welded in for 02 sensor, egt sensor and pressure sensor etc because welding them on once it has been coated is a bitch.
You can run it all in aluminum, make the first bend or two in 11 gauge then switch it to 16 gauge. My 5" is all 14 gauge and never an issue but I don't recommend that to everybody, I can try things without getting anyone mad at me.
You guys sure it's 6061 ? It can't be bent in the T6 form unless it's annealed. Then you loose the benefit of 6061. Just thinking out loud.
I have tried even 14 right at the turbo and it cracks. But these were tight spacer where it had to bend RIGHT out of the turbo, which I think was the cause. Did SS for the first two feet and then 16ga for the rest in 6062 and it's been good. Polished the SS first and it turns a gorgeous gold when it gets hot. This pic doesn't do it much justice. But you can see shortly after the bend, the v-band transition to 16ga AL.