Lowering Suspension

I think lower suspension can make a car feel more stable, but it makes it feel slower, whereas a stock suspension will be stiffer but possibly more rewarding.

I'm probably not the best person to ask, but I believe it is just a feel thing, not an actual performance thing. But I guess if you can make a car feel better, then you can push it more. On most cars I have stock suspension, but on certain cars I find difficult, I may lower it a bit more.

Honestly, you should only start to worry about this stuff when you're at quite a high level of racing already. If you can get around a track consistently without crashing, while also hitting roughly the same laptimes, that's when you should think about advanced mods for your car imo. 

Also, this video by Broughy explains what each aspect does as well.

I choose the middle suspension, no reason other than best of both worlds and if 1 is better than the other so be it!
Middle I go!!

CHILLI made a nice document with loads of information about car handling and such. You can find it in this thread.

I noticed from my couple years of experience doing gta racing, that lowering the suspension on oversteering cars made the car easier to push with because of higher stability which is again because of a lower center of mass. Usually I go with the highest possible controllable suspension to have a feeling of maximum responsiveness while also still having control over the car.

For a car with an understeer-y nature like the Massacro, I can advice to use stock suspension.

For years I always lowered the suspension. But now that I care more about the style of the cars, I look at them and honestly most cars look better with stock suspension. I noticed this right away when I brought the new Toros, I lowered it like I always did hen looked at it and was like "man... this car looks like shit lowered" then I went back and started taking a good look at my super cars, and cars like the X80 and such, then honestly all look better with stock over being lowered.

As for handling, I don't really feel a difference. I most race with randoms doing stunt or transform tracks, and sometimes they'll lock the cars to stock. There honestly isn't much difference between a stock X80 or a custom one. I mean yes there are obvious things, a stock car won't have turbo and will be slower, and it'll have a little less traction due to no spoiler. But in general a stock car feels the same as a modded car. Far as suspension goes, in any stock car races I've been in I've never felt like my car might flip over because the center of gravity is higher.

Then only time I ever noticed suspension to make any difference is on really tall vehicles like the Maryweather Mesa or Sandking. You can choose to lower their suspension and they do feel a tad more stable, though personally these I still leave stock. They are far to slow to win a race and I use them as offroad joyride vehicles, so I want max ground clearance. But far as super cars, sports cars, etc, honestly I don't feel like it makes any sort of difference.

Even broughy has said the main things you need are engine, brakes, transmission, turbo, and spoiler. Suspension is something that makes so little difference that it'll only really matter to absolute pro racers looking to draw every little milisecond of lap time from a car. Any average slog or even moderate driver, is probably not even going to be able to tell the difference.

Vucko100 testing pretty much supports this as well. He finds that lowered suspension might help in corners, but it's to such a little degree that it's within a players margin for error. And oddly enough, his video actually kinda supports TC as his car with the lowered suspension tended to have a little bit less straight line speed.