THough im no expert i dont think being NA and lacking back pressure has anything to do with the sound being emitted. I'd say its your muffler that is now working to muffle the sound where as the cats use to act as mufflers too... Especially a glass pack style muffler with staight shot through would produce a rasp like "bat bat bat" sound.
you need approximently 2 cfms (think its actually 2.1 but cant quite remember) of air to support 1 horsepower.
If you have a 2.5" diameter pipe with a high flow cat that flows 360 cfms, it can support 180 horses with 0 loss. If you increase the horsepower you'll see around a 10% loss or 1hp loss for every 10 gained...
Important to remember exhausts dont "add" horsepower, they "allow" horsepower.

A 2.5" carsound cat supports 360 cfms.
A 2.5" random tech cat supports less. think like 330
to give u an idea of how well they flow...

Its very likely your stock cats flow much less than 300cfms.