if the primarys are long then 1.5 inch will be fine upto 500bhp , but if you used a larger tube like 1 5/8 with short primarys (like a focus rs manifold) then you would probably gain a few bhp but the torque would rise .
i use on my over gearbox manifolds 1.5 inch primarys , 1.5mm thickness and 10mm thickness stainless steel flanges both header and turbo flange
even the 10mm stainless flanges are bolted down during welding to stop the flange from twisting

hope this helps