Not much, then they'd upgrade the flywheel, then the CV joints, then the gearbox, then the engine mounts, then the brakes, then the tyres, then the suspension, then....etc.
Most cars are designed with a set of parameters and load ratings for their components, starting with the engine then cascading down the drivetrain. Upgrading one component in isolation often affects others e.g. fitting wider / low profile tyres increases wear on suspension joints and bushes.
You are correct in your assertation, cars are built to a price and the manufacturers make sure the car will last for about seven years doing what it's designed to do - the problem with this is when an owner makes the car do something it's not designed to do, such as increasing power or towing a heavy load at high speed over long distances.
Most cars simply don't have the spare capacity to perform over and above their normal task, and the ones that can are invariably at the 'premium' end of the market (and no, I don't include Audi in that group). Fiat small cars generally fare better because they are lighter, they're not used for towing and performance gains are usually much smaller.
Every extra HP or NM produced has a cost somewhere down the line - it's just a matter of what gives up first. The car in its standard trim is fulfilling its design, if you want more power & torque then you either get a different car or accept that drivetrain components will have a shorter life.
I'm not having a pop at anyone, so please don't take any of the above personally, I'm simply drawing on my thirty years of vehicle / aircraft engineering experience. Ten years ago, a Fiat Brava had a 1.9JTD engine putting out 105BHP in a car that weighed 1195Kg. A Croma 1.9JTD puts out 150BHP in a car weighing 1530KG. More is being asked of drivetrain components than ever before.
With modern design techniques, you can be sure that every component in the drivetrain is 'fit for purpose' and nothing more - and its life will be limited...
HTH.
