I am not surprised the 2.5 inch pipe hurt your performance. Its just too big for that small of displacement engine. Id go back to 2.25 or between
that and stock, and change to a better flowing muffler for starters.
Ideally, you want fast exhaust flow from not to big a diameter pipe...just big enough to handle the volume of your tuned up engine and NO backpressure from the muffler or as little as you can get

 

We are talking about crush bent exhaust if we weren’t I'd completely agree with you. Your exhaust is only as big as a the smallest bend in it. SO if you are running 2.5' crush your basically running a equivalent to 2.25 mandrel bent piping. Crush bent has too much turbulence to have a smooth flow or sound to it.