not very
yes
possibly
yes, with enough money you CAN polish a turd. just ask the civic guys.
If you think about it, import tuners aren't doing anything different than teenagers in the fifties were doing - taking a car they can afford and trying to make it go faster. Problem is, there are a few who are more concerned with the look/lifestyle than actually trying to build a car (Vinyl adds Horsepower dood!). A giant chicken wing on the back of an Accord with a K/M air filter and MSD ignition
probably isn't very functional. But you can take a Civic, swap in a B-series motor, put on some forced induction, gut some weight, and do some other work and you'll have a fun car with some serious HP.
In my limited experience though, GT is pretty good when it comes to HP gain. The 2 big enemies of a car are gravity and friction - do things to limit those (increase airflow, lighten the weight, reduce moving parts, etc) the car will become faster. GT does a decent job of addressing these.
Bolt ons will increase horsepower but not as much as some good internal work. Obviously, GT simplifies this process - I doubt
most people would want to shop individually through multiple kinds of pistons, camshafts, carbs, valves, etc.
