Going from a 2 barrel carburetor to a 4 barrel that's properly tuned can get you up to 25 horsepower, proper ignition timing can make a extra 20 horsepower, if you go from like 0* to 16*-18* btdc you will gain more than 5 horsepower, you say only 2 on the exhaust, you must be installing your exhaust system with a hammer, if you properly install a a good free flowing exhaust system that's mandrel bent it should be good for a easy 10 horsepower. I wish that I had the dyno sheets that showed power gains with correct timing. If want you say is true then going to a 4 barrel would be a waste of time and money.