Not many exhaust shops will build a proper mandrel bent exhaust but will gladly take your money to make something with crush bends, aluminized steel that is more prone to rust, often very questionable quality materials, poor flowing mufflers and or resonators....simply not worth it when for a bit more you can buy a very well made SS system.
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Comparing the gains on a big turbo Supra to a tiny turbo FiST just does not equate.
Stock exhaust is fine for most owners, Cyborg can use a bigger setup and a 2.5 might be better than a 3" but only real testing would prove which is best, big turbo, 3" most likely best but not absolutely, it could depend on many other things to a particular setup.