My 2002 Olds has 200,000 interstate miles on it, and used to get 32mpg, but now gets 26-28mpg. Sometimes when starting, it doesn't seem to want to start. The only that has been done is a new fuel pump at 98,000 miles, and someone tried to steal my catalytic converter, so it has been welded back. After the fuel pump replacement is when I noticed the decrease in power and lower mileage. It's a 5 speed, so I can get power when I need it, but it doesn't run like it used to. Is it just the high miles or is there something I can do to boost performance? What about a superchip or adding headers -- any suggestions?

 

I assume the fuel filter was replaced at the same time the pump was, if not, it's due.
As far as the mileage issue, check the usual culprits: tire inflation, air filter, Throttle body gunk, MAF sensor gunk, IAC valve gunk, EGR valve gunk, etc.
I was surprised at how much buildup there was on the back of the throttle plate at just 95K on mine.
I'm guessing that you actually compute mileage, if so, when did you start to notice a decrease? The change from summer to winter gas can affect mileage, if the stations in your area do that.