So I'm a rather big car head. I tested everything with and without upgrades. What I found is not everything affects the stat bars. Here's the break down of what I found.
Engine upgrades affect the top speed-ish. I think every car has a set top speed and this simply affects how close you can get. Even when the car sounds like it's topped out (max rev with no gear change) it's still gaining more and more speed.
Transmission makes gear changes faster from what I can tell. They also increase acceleration.
The turbo should only be installed on certain cars. Any car that has wheel spin stock should NOT have a turbo as it only increase the wheel spin, unless you're looking for more wheel spin. If you own an AWD car, install that turbo, it works wonders in AWD cars.
Wheel types DO affect traction! While they don't affect the stat bars, each wheel type has it's own personality.
High ends give good grip at high speed and average at low speeds.
Low riders and muscles both give lots of wheel spin and are loose through corners (great for drifting).
Off roads don't really affect traction in dirt, sand, etc but they do have superior grip in the rain. Sports are your generic all weather tyre and and provide mediocre grip in all conditions and situations.
Tuners are weird. They are give a bit of wheel spin, but when they catch you'll jump away. They're extremely good for lower speeds and okay at higher speeds. They're also my choice on my getaway vehicle as they loose grip easily, but gain it back just as quick.
Suspension upgrades lower the car, and lower the centre of gravity. This allows for better traction through corners. Irl there's a lot of factors that go into this, but in gta v there's not much. The car WILL stay level through corners no matter what, unless you hit something. In leymans terms, suspension = better grip though corners.
Wheel upgrades do not affect traction and grip.
Brakes don't affect the car in the way one would really think. Brake ipgrades simply add an ABS system to the car. This means the wheels won't lock up when you slam on the breaks, this decreasing the stopping distance. I tested stopping with and without the upgrades and the distance was about the same. These upgrades are actually pretty pointless if you know how to stop without an ABS system.
On the few cars that have them, he roll cages don't affect handling as far as I could tell. They're simply aesthetics.
All the other upgrades are simply visual and do not affect control of the vehicle.
Turbo: Dependent on vehicle
Wheels: Dependent on driving style
Brakes: Optional if player is skilled
Remember that each car has it's own personality so you should always cater the upgrades around that and I promise you that the car will perform tones better.
And to answer the original post, if the stat bar is maxed out, it will still increase performance.
Edited by Belwrath, 4 weeks ago.