because a 1967 Ford mustang GT370 thats supposed to be running 300bhp...... after just 40,000 miles had just 215bhp.
you have got to consider piston rings, valve seat wear, bearing wear, "coking", General Fatigue of component parts..Regardless of mods.
lets say that you have lost approximately 15-20 bhp from standard over the years.
thats 35BHP, then add your mods, which to be honest on a tired car arent gonna give the same gains as a new one..... so lets say 3BHP for both the k+n and the exhaust, thats still only 41bhp...
i could possibly say about 60-70bhp on twin carbs or a masively flowed head, but it would need some serious maintainance to achieve this.