Headers won't give you the seat in the pants power till your flooring it somewhere, passing etc. Part throttle ispretty much the same although everyone's experience is different. There definatley worth it if you're upgrading other things as well.

You will get a louder exhaust without the cat. The "power" band feel also shifts up slighty. You will get some better highway mileage as well.

Remember though you're not deleting the cat without throwing a code you will need a tune