Headers vs 340 manifolds

ir3333 said
If you were able to increase a stock 340 by 15hp by adding headers, could you feel the additional 15 hp in a 3600lb. car?

Or look at it this way hp is 57% of torque at 3000 rpm so 10-15+ hp at 3000 rpm is 18-26+ lbs-ft at 3000 rpm, is 20+ lbs-ft down low worth it ? It's basically like adding 20 cid.