so the global decision is a plenum spacer and forced induction?
i dont think there is a "global decision" for every mod. thinking about a plenum spacer and F/I are in 2 completely different ball parks.
What do you want out of your modding? are you modding your FX for fun, 1/4 mile times, auto crossing, or just for having some fun on the roads?
how much money do you want to spend? a plenum spacer costs around $300, F/I will cost you at least $4,000...think about how much you want to spend and then go from there.
if you are looking for your best bang for buck...a 5/16" or 1/2" plenum spacer is a great addition, a z-tube with either a cone filter (stillen, jwt) or a better drop in filter (k&n). a crank pulley is a cost effective mod and i have personally felt good gains from it. test pipes really free up your exhaust but make things louder and your exhaust will smell more....
most mods have many points to them, e.g. test pipes. a lot cheaper than high flow cats and they provide very good gains for n/a and f/i. although they are not smog legal, make the exhaust louder, and there is more smell. think about what you want and you can see which mods are right for you.
btw..i would ditch the injen if you haven't gotten it already because it has been dyno proven to LOSE power. (this is all on the .org, just do some searching). most reputable companies provide mods that give good gains.