Just FYI, I had a ported top end on a stock exhaust. Changing the exhaust yielded ~25whp gain. Then adding headers added another 20whp.
The stock '90 system is pretty bad, especially the resonator. But if you have a Borla system, even if it's not your fave, it is likely better than the stock system. So I'd say headers. But if it's a stock exhaust, I'd say exhaust.
