Exhaust system

I have a 97 - 3/4ton 12 valve CTD auto trans. It's a basic "stock" truck. The only thing that I've added is a trans. temp. gauge and a K&N air filter.
I talked to someone who said it's great to get more air in but you have to get that air out too. He suggested I install a 4" exhaust system. That's a $500 bill! Would I notice a performance increase if I were to do this? Does this increase the HP, or better mileage? What is the benefit of doing this?
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You might want to rethink that K$N filter: http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm
Budd

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