What pipe material/thickness for a rear turbo?
 

Plan is to go back to coated headers and wrap the pipe to the turbo. Also going to 2.5 in diameter to keep heat/flow back to turbo. What economical pipe material/thickness should I use?

I hope by headers you mean stock manifolds, because individual tube headers will make your heat loss problems going to the turbo far worse. The heat loss in a rear mount system is an unavoidable issue that hurts power and spool time, so do everything possible to minimize it. As for hotside pipe material, I would use whatever cheap exhaust pipe is available, but then use the best heat wrap material I could find all the way back. I can tell you from personal experience that the results will still be far inferior to a properly built front mount setup, but I understand that you only asked for advice on how to do the rear-mount.

Most economical will be aluminized pipe, best would be stainless but that will be $$$ and heavy.

Check out this older thread of mine, might give you more insight on what works for a rear mount.

https://ls1tech.com/forums/dynamomet...y-sts-kit.html

What is this going on? I would consider 2.25" pipe also.

Yes going w/ coated stock manifolds. Saw other rear turbo posts that 2.5 will work just fine and spool quickly. Will y pipe w/ high flow cats help or hurt?

Any updates or info on this ?

High flow cats will hurt quite a bit when pieces of them go through the hot side of your turbo. It is not a matter of if, but when this happens. This is another downside of rear mount setups. If you make them emissions friendly they will eventually self destruct.