go ahead. mod it, its 4,000lb car. But if it has the L67( series 2 supercharged), you can make power.
my father just got a 2003 ultra, the thing isnt as slow as i expected. the airbox is actually very good out of the factory, all u need is a drop in filter and ur good to go.
i suggest getting a mild cam, ported manifolds, smaller pulley, and tuning work.
i dont think headers would work b.c of how its setup. unless u can get a custom downpipe to bolt up and match ur exhaust.scan you car to see if u have any KR, and go with colder plugs if u do.
this setup will make u good power to kill some of those imports.
i have a set of ported manifolds for sale if your interested.