Well there are lots of factors that affect mpg beyond the emissions systems. Did they have manual or auto trans, what was the final gear ratios, what was the coefficient of drag, were you driving at the same speeds? Driving faster really eats away at MPG. Etc, etc, etc.
Because the engine control systems have become better over the years (more powerful CPUs, sensors, algorithms, etc) less raw pollution leaves the cylinders than ever before. This lessons the work that a catalytic converter has to do and allows them to flow exhaust more freely than in years past. Yes, restricting exhaust flow in any fashion will increase backpressure and lower the thermal efficiency of the engine, but I would contend this is not a significant loss anymore. Other factors have a bigger affect on mpg.
There’s not a whole lot more that can be done to improve gasoline engine efficiencies. There are no more magic bullets left in the engine. Most of the gains in the future, to get to Obama’s new 54mpg CAFÉ law, will come in the form of smaller profile cars. Get used to it. Small is in our future!