Just the facts: you have a 98, with a 2.5 liter engine. It's the smallest Boxster engine and you won't wring much more performance out of it. Maybe an underdrive pulley would provide a little increase. But it really is what it is.
You can get better tone with any aftermarket muffler, and they can be dirt cheap. I have this one on mine, makes it sound like a fire breathing dragon. Looks cool, too.
https://www.amazon.com/gp/product/B0042LLELW/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
The thing with your car as it sits, it probably runs well and is fairly reliable. Getting spendy and fancy will just make those attributes go away. You'd do well to make it fun to drive, comfortable, and pretty.