Every car on the road is a product of compromise.
The Fiesta ST got a custom interior but its only about 50% finished in my opinion, they left out a bunch of details that are not that hard to finish off.
The suspension is nice but $2,000 for very nice coilovers was not in the budget (and won't upset the balance if valved properly).
The engine is nice but an old 1600 twin cam from the 70s can rev to 9,000+ while our 1.6 only does 6,350. A bigger turbo and fuel system should allow full boost until at least 7,000 with a lot more power. More power at high rpm won't upset the balance either.
The brakes are typical OEM stuff, inexpensive and designed for ~200,000 mile durability. An upgrade can reduce mass, increase brake torque which you can use if you get sticky tires and won't upset any balance.
You can mess up the balance of a car if you change the roll centers, roll stiffness, steering geometry, add too much grip on one end or just screw up the alignment.
If you upgrade correctly, it will just be better.