Well sound can be changed substantially just by replacing the cat back section of the exhaust. And with a better flowing system, performance will be improved. But speaking from my own personal experience, I think a COMPLETE exhaust system (headers, x-pipe, high flow cats) would be well worth saving for and doing. It will make a NOTICEABLE improvement!
If you want to take small steps, try to have an overall game plan in mind so that any minor mod you make won't be wasted if you want to take a bigger step later on. No sense just wasting money, so do some planning first.
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