if they want 4 to 450 to make and install a 2.5 mandrel bent cat back how is that 3 inch set up not cheaper?

buy a 3 inch res or glass pack, then 3 inch mufflers, and take it all to the shop, now its just install labor, no parts needed from them.

hell you could do that install in your driveway, its all slip joints, the only welding is the hanger bars on the pipe to the under carriage of the car. you can clamp every slip joint if you wanted to.