Here is a customers very mild 2000 Suburban with a 5.3L.  Only modifications are an air raid cold air intake kit and a cat back exhaust.  If you compare these dyno numbers with my bone stock 1999 5.3L project truck you get an idea of the gain from air intake and exhaust since they were both on the same dyno for a good comparison.  I overlayed the stock and after graphs so you can see we got a decent gain at all rpms both in horsepower and in torque.  We had a small flare in the graph at the beginning on the stock dyno pull which gave it a higher than actual torque number.  But check out the graph.  Stock peak was 221hp and after was 238hp for a 17hp gain and torque is a little harder but looks like about 255 versus 268 for a 13lb torque gain but the torque curve is a bit flatter with the new programming.