A couple of things that need to be understood. Just because you "smash" an exhaust pipe it doesn't necessarily hinder flow. That's the most important thing and it only applies to a point. Ovaled tubing can flow exactly the same as round tubing. Once yo pinch it beyond oval then you have issues of course, the pipe in question is nowhere near that point.
The other thing is, that pipe only flows 1/2 of the volume the engine needs to expel. For a bolt on truck, that "smashed" pipe is absolutely 100% fine. Leaving it won't hurt anything just like changing it won't add anything. It's more than capable for his mods.
FWIW there are numerous calculators online for calculating the exhaust CFM needed for any engine. Do the math, divide it by 2, compare to the stock size tubing and see what you'll learn. You'll need intake CFM also, but that's an easy calculation too based off of engine displacement and RPM.