"power adders" I'd like to mention
m112 supercharger - love it. Pretty effective. Negative is size, but thats also a positive cause it looks intimating. Also cooling it is a negative.
love superchargers and force induction in general
Meth - love it. lasts a long time and is effective. No negatives... so far.
373s - love it. really nice driving ratios. best bang for your buck is gears. Only negative is still havent figured out how to launch in first effectively.
Mac catback - not sure how it compares performance wise, but best sounding setup on v6s hands down
I dont know what else is worth mentioning.
ADD:
I suppose i could mention engine swap, alot of this could go for most engine swaps;
Positive: 4.2 swap is worth it. Is a pretty good upgrade and works well. Split port and the larger displacement are very nice. Like all engine swaps, you swap because it'll be worth doing the work upgrading over the previous engine.
Negatives: at first you will likely have a few bugs. Its a pain to work through them, but eventually you'll get through it.
Another thing that is annoying is trying to figure out which parts you need while building or repairing. With a setup like mine where there are multiple vehicles involved you really need to know what part is for which. You can't go down to the part store and explain which part you need, theyll have no idea. You have to be like "I need spark plug wires for a 2000 mustang" even though its for a 1994 mustang.