Over engineering even in the design stage usually means huge costs. In my experience, the cost driver is usually a big enough one to remove alot of over engineering. Of course, the components and the system would have been designed to a specification, and that spec would have had performance tolerances. Unless they deliberately designed the engine for something like 500 bhp, I dont believe that they would have then spent the time and the effort testing everything to a lower performance metric. It doesnt make too much financial sense let alone engineering sense!
Durability for me means that my supe gets me through the 12000 miles it gets driven per year without breaking. I have tolerances for things dying on me as its a 10 year old car. I am surprised by the reliability of the car, and have been impressed by the tuning I have carried out.
As we have all seen, once you start tuning you move away from the engineered equilibrium that Toyota envisaged and designed. You remove the cats and have to alter the fuel cut, manage the boost creep, put up with increase fuel consumption etc...
Its the same with any system... it has been designed for a specific purpose but invariably with built in tolerances. Exceeding these will not mean an instanteaneous failure, more that the chance of failure is raised to an unacceptable level (one which would not pass a quantative testing process)