Your car seemed to be slower with the 499 that I would've thought it should be also so perhaps you have a car problem and not an engine problem. Have you gone over the car with a fine tooth comb? Any chance the brakes are dragging? Maybe a fuel delivery problem? Maybe the torque converter is bad? Have you been using the same gas the whole time? Maybe a drum of gas that is a few years old or something? Mufflers plugged up? Lots of stuff can go wrong in a car so it might be worth checking it over before blaming the engine.