Mod the muffler if you don't plan on making the engine louder.
My stock muffler will whoop your 40 or 80. hahahaha Darn thing keeps getting louder by the mod.
Your best mod right now is putting the maf screen back in. That'll straighten your airflow and stop you from running on the edge of lean. Being a bus boy, you probably don't want to spend the money to replace a motor. Most folks notice a gain from removing the maf screen. Unfortunately most do so w/o scanning and tuning. This means that the calibration of the pcm vs how much air is flowing is off. Therefore the entire fueling calculation is off and causing the car to run lean. Lean is fast..fast is hot and hot and lean blow pistons to shreds.
After the maf screen. I'd probably port/polish what I could while doing LIM gaskets and maintenance items to keep the car up to speed and running well. Port and polish while the LIM/charger are off. Things like that.