Legal or not
 

Is it legal to add an aftermarket exhaust on a car that is not fully paid off yet since the bank technically owns it?

Lol yes. 80% of people on the site would have illegal exhaust if that was the case.

WTF?!

You own the car, the bank owns what the car is worth. So if you can't make payments the bank repossess the car so they can sell it to get the money you owe them. Do what you want to it...

Interesting

It is the same as when you buy a house on a bank loan.
You can modify and improve your house as much as you want.
Same goes for cars,

smh... scion questions.