Done a lot of header swaps over the years, and honestly, have never seen a notable performance increase on a stock mild performance engine. Tubular header install along with a cam swap, induction change, full exhaust, yes, absolutely. On the positive, once the install is done additional performance mods will be easy to do. A stock NA engine can only reach so much VE. As packaged from the factory, maybe it might be in the 70- 75% range (you might be able to find dyno sessions data on the net) modifying and tuning might get it to 80-85%. A well tuned engine built for racing can get into the 90%+ range, and of course adding boost is required to get above 100%.