If you think of anything else down the road I'm all ears.
Regarding power - well there are a few different ways to measure that but for the most part we're concerned with power at the wheels (a number you look at on a dyno) or the way the power feels (driving pleasure). To some people those are the same thing. I find that a little disappointing but if that's you, well.. just get a different car. Sell yours before you spend any money on it. There is nothing here for you. I mean this in the most positive and helpful way possible.
As for getting some fun power out of the motor, get used to the idea that there's no reasonable way to make 200 horsepower on stock single port heads. You can get close though. Numbers are stupid.
The first bottleneck is the single 2" exhaust. From the Y back. The cats are not a bottleneck. None of the emissions equipment is a bottleneck of any significance. You'll pick up real power you feel with your butt dyno with dual exhaust or a single 2.5" exhaust even if it's not any louder. Sound isn't power, weed wackers are loud AF too. Pacesetter makes/made a true dual adapter that keeps the cats but eliminates the Y, it's awesome and then you can run any GT catback you want including a used takeoff.
After that you have a bunch of different places that represent real restrictions, in no particular order.
The heads don't breathe well so if you want to just attack that head-on you can either spend more than you spent on the car having the heads ported or you can get a set of split port heads from a new edge or a van.
Or you can get a whole 4.2 from a truck and drop it in, it's the same block. Both of those jobs are fairly significant because the intake is different which means you have to do a little fooling around throttle linkage, egr, wiring, etc.. but if you want to just start making 200+ or have any chance at 250+ horsepower without boost, that's the only way to go.
If you don't want to take the heads off or swap the whole engine then you have to work with the heads you have. You can get more air into the cylinders a couple different ways (or all of them).
These engines were made from the very beginning to be inexpensive and to return decent fuel mileage for their time with more punch off a stoplight than a 4 cylinder, for sedans/wagons/pickups that might have to carry more than just two people. They don't have variable cam timing or variable intake geometry so that means they have one BSFC tuning peak and that is at a very low RPM.
The two things which drive this low tuning peak are the intake runners and the cam.
So to raise the tuning peak you enlarge the diameter of the intake runners (all the way from the plenum to the port) or you shorten them or both. There's math you could rely on if you were actually going to involve yourself in some sort of sad racing bracket for cars with unloved engines but you can safely ignore that because there's not enough aluminum there to move the tuning peak outside of the necessary rev range. Just port them as wide as they'll go and if you can cut and weld aluminum cut the plenum off and nibble down the ports from the back side, then weld the plenum shut again.
You can run a cam too, that keeps the valves open longer and taller than stock. The stock rockers are 1.7 so make sure your cam is compatible with your springs and don't buy more cam than you can afford in support parts (roller rockers, different springs, etc)
You don't have to do both of those things, but you can. Depends how much effort you feel like putting in - remember what I said up front if you really want a big impressive number to look at and tell your friends you got the wrong car sell it and buy what you really wanted.
After you have exhaust, and gears, and have done whatever you're gonna do with your intake manifold and/or cam (if anything) then you might want to look at a tuner to raise the rev limit, maybe 19lb injectors or an adjustable fuel pressure regulator, maybe underdrive pulleys (again, AFTER exhaust and gears)
The throttle body and the filter/maf/etc assembly are not bottlenecks as long as you have stock single port heads, you could give people money for different ones and maybe hear more induction noise but not change your power by any kind of amount that relates to the money you spent.
I wouldn't worry about stiffening the chassis it's not floppy. The floor pan is a little flexy but the chassis isn't. You can install the strut brace from a GT over your single port manifold nice and clean, it's worth doing in my opinion - but that's less of an option if you run a different manifold. There's a lot to be said for enjoying a clean, good running car and not caring that somebody else somewhere thinks they're hotter sh**. A good set of shocks, new coil spring isolators, new front control arms (because your original ball joints are time bombs) and new rear control arms (because your orignal bushings are bubblegum) will go a very, very long way toward making the car feel great to drive. Don't spend too far into the future start with the stuff you already know will be rewarding.
Being real a lot of the pokemods people insist on collecting are a waste of money and I've seen way too many people get a car they're never going to like (a V6 SN95) and convince themselves they need all these dumb things because it's in the catalog so it must be great... spend a fortune, flog the crap out of the car because they're mad at it for not being a GT350, then sell their beat up disaster to some other poor jerk at a loss or beat it up so bad it goes to the landfill. I can't really say anything good about that.
