Let me start off by saying. Your car. Your money.
I'm one of the ones that have walked the walk. Poured a ton of money into a new 1994 Cavalier Z24 which I daily drove. I was young and just starting out with getting into cars and the whole tuning thing. There wasn't really any part of the car I didn't touch. Engine wise I did a Crane Compucam 2030, ECU chip, Crane 1.6 roller rockers, custom clutch, custom cat back exhaust with free flowing cat, and nitrous injection. There were other minor mods. Suspension wise I did Eibach Prokit springs, Energy urethane bushings, caster plates, Bilstein struts modded to allow for more camber, Koni Sport rear shocks, and larger rear sway bar.
So there was quite a bit of work done and the car was a total sleeper. Would I do this again? Not so sure. I learned a lot with this project which I carried over to my later car purchases and what I did in terms any mods.
So the problems. Even though Crane said the 1.6 roller rockers were a drop in, I had to have the lower intake manifold modified by grinding certain areas down to allow for the clearance needed. The valve covers had to have the insides machined down to also allow for the clearance needed and the rear valve cover required the PCV opening to have a baffle welded to it. This also required a machine shop as welding aluminum isn't something you would have your local exhaust shop just whip out. The rocker studs also had to be slotted because the amount of clearance wasn't there to install them. The custom clutch I got also had clearance problems requiring the manufacturer to send me a different pressure plate from one they thought would work. I went through 2 different tuning companies in trying to get a chip that would work properly with the car's ECU. One had a novel idea where it allowed me to switch between their tune or the stock tune with a literal flip of a switch. Never worked despite several attempts with updated hardware. The second company initially didn't work either. After some R&D on their end, they finally got it to work with a piggy back set up where the chip would be installed in the ECU and the stock chip would connect to a piggy back connector.
Along the way I've had some major mishaps. I went through 2 blown engines. All due to being under spray. First one was a NOS fogger system. I didn't go beyond NOS' recommended jets for my car. Yet a piston cracked which seized the motor. We all felt it was a manufacturer defect in the piston which only came to light when I pushed the engine under spray. All the other pistons were fine. Second blown engine was a result of a nitrous solenoid malfunction. I got off the spray but the nitrous solenoid jammed open. This was with a Top Gun nitrous system. I ended up going back to nitrous again but used NOS' dry spray system. This time things were fine and no other issues.
Because of the less than ideal roads where I live and with my suspension having the ride quality of a go kart, I started to see failures in the suspension components. The rubber bushings in the inner tie rods went. The Bilstein struts failed due to the pounding they got from the pot holes and uneven road surfaces. I've had multiple failures of the half shafts at the CV joints. The trans went on the car requiring another trans. This was due to the clutch system I put in the car. The hydraulic clutch system didn't have the pressure to fully disengage the pressure plate as I got the custom clutch for more clamping force. So when I was disengaging the clutch, there was still some engagement happening which wasn't good for the gears/synchros. Also with the clutch system, I went through a couple of failed release bearings due to them being stressed from the pressure plate.
If you've gotten this far, good for you. Here are my lessons learned. It's fun and some what satisfying to have something unique. I sure as heck did as I only knew of one other person in the country that had a better modified Z24. When you modify a car so extensively which is only meant to be an econobox daily driver, you're going to run into lots of issues. As you can see, I did and it wasn't from a lack of buying quality parts or half assing installation. The mechanic I used was a ASE Master Tech. Owned his own shop and has had extensive engine building experience from doing twin turbo Corvettes to Camaro/Mustangs to eventually being the recognized SHO (Yamaha engine) expert for the entire east coast. I'm sorry but those saying they're going to take a car which only has 168 HP from the factory to 500 to 600 HP AND daily drive it are fooling themselves. When you modify a car that extensively, it becomes a project car which only gets occasional use. So one of the lessons I learned is to have a daily beater which is a daily beater as a second car. When I had my Z28, I had my Z24 as my daily which eventually was done in by the number of miles I had on it and a blown head gasket. I got a Ford Focus ZX3 as a replacement. Now I have the C-HR. The Focus and C-HR's mechanicals are untouched. My current "project" car is a BMW 135i.
The amount of money poured into making an econobox go fast is better spent else where. There are other cars out there that have the foundations to be modded more easily with better power results. I keep hearing I can pour X amount into said car to make it just as fast or faster than Y cars. But if you took the total amount spent (X amount on mods plus price of car) and picked a more suitable car where the cost of mods equals the aforementioned budget, you would end up with an even faster car.