The funny thing about your car's engine, suspension, brakes, and chassis is that the order in which you ought to modify it all will never really change. Bolting on a three-inch exhaust before getting rid of the factory airbox will never make sense nor will fitting all sorts of impressive chassis braces and anti-roll bars underneath when you haven't even bothered to spring for a decent set of tires. Read on and make sure that your next build doesn't follow the path of the bonehead.
Springs and Shocks or Coilovers
For decades, dumping ride heights by means of lowering springs or some sort of adjustable coilovers has been the starting point for almost every build. There's a reason for that; stuff a big ol' exhaust on the back of your bone-stock Nissan with four fingers worth of fender gap and it's obvious just what a tool you'd look like. A lowered ride height doesn't just make everything look better, it drops the car's center of gravity and, in most cases, stiffens up the suspension enough to allow for better traction and handling.
Wheels and Tires
Few performance upgrades are as beneficial and as obvious as the right wheels and tires. Yet too often heavy, weak, low-quality wheels cast by children in third-world countries stuffed inside of tires you've never heard of are used. It turns out that higher-end forged rims and tires with the sort of compounds that were designed for the sort of driving you plan on doing will have a bigger impact on how your car handles than just about anything else you'll ever do.
Chassis Braces and Alignment Kits
Save those new tires from getting all bungled up from those camber and toe changes introduced by your new lowered ride height with some sort of adjustable alignment kits. Now's also a good time to bolt on some sort of chassis bracing, although, with most newer cars, such mods often aren't all that noticeable.
Working backward will almost never make sense, and the path air takes to get into and out of your engine will never change directions. In most cases, the factory airbox and intake tubing poses the most significant and obvious restriction. Yep, you might have to sacrifice that mad-tight, JDM look after you've ditched the stock plastic airbox but your engine will be better for it.
You've been waiting to bolt on that obnoxiously loud exhaust since step one but, in terms of naturally aspirated applications, at least, it won't make a whole lot of difference until now. Until the intake side of the engine's been opened up some, increasing exhaust flow won't be all that necessary.
It's true that your engine's header(s) are stuffed in between its intake piping and its exhaust and, all of a sudden, we're not following that same path, but there's a reason for that. In many cases, headers won't yield as dramatic of an effect as an exhaust system does and simply won't give you the sort of results you wan't without opening up what comes before and after it first. For obvious reasons, disregard this mod if you've got some sort of turbocharger underneath your hood already.
Clutch and Flywheel
By now you've either added enough power to warrant a more durable clutch or you just think you have and you've toasted the stock one anyways. Either way, the right clutch will allow you to better transfer all of that torque to your tires, and a lighter flywheel can make all of that happen just a little bit faster.
ECU and Electronics
You won't be able to show off whatever tunable ECU or piggyback box you've just wired into place at your next donut shop meet but, like the right wheels and tires, it'll be one of the most important mods you'll ever make. That intake, header(s), and exhaust you just bolted up? You'll never maximize all of those gains without some dyno time and a little bit of tuning.
Cams and Valvetrain
Once you've popped the valve cover(s) it's safe to say things have gotten pretty serious between you and your engine. Bigger cams and more durable valvetrain bits will do you little good without first opening up the intake and exhaust paths and having some sort of tunable ECU to make it all work together, though.
Now's about the time to chuck whatever intake and header(s) you've got or ditch your factory turbo system. No other mod will give you more power than a turbo or supercharger kit, but without the right exhaust, clutch, and tuning solution, you'll be destined for failure.
Forged Bottom End
Not blowing up your high-horsepower turbo engine is almost as enjoyable as experiencing all of that newfound torque. Depending on the make and engine, forged internals, like pistons, rods, and even more durable cylinder liners, could be necessary as early as 250 hp.
Also there to keep you from blowing things to smithereens, a higher-flowing fuel system made up of a bigger pump and injectors has got to happen should you ever want to crank the boost up.
Being able to stop is almost as fun as being able to accelerate as fast as you think you should. Most of the time, larger rotors and calipers with more pistons than what the factory already set you up with won't make a whole lot of difference if the ability to make a whole lot more power and you needing to slam on the brakes in a hurry and without overheating them isn't there.
Besides tires, few suspension mods are as noticeable as the right anti-roll bars. Typically, anti-roll bars are there to balance out an already put-together suspension. Hold off on these until springs, shocks, and ride height have all been configured.
More than likely, your street car doesn't need any sort of aerodynamic help beyond what the factory already did for you. You'll never be going fast enough to take advantage of any of it, which makes it the obvious upgrade to save for dead last.