Lol everyone says reliably until "Bang" catastrophic failure!! I love to see how far people can push the stock internals but its only a matter of time before something gives. My goal is 300whp on stock internals but I think too much more than that and you're asking for trouble. Just my opinion...
Depends on the motor. Reliability and longevity of a motor will definitely be shortened when you demand more power from it, that is absolute.
There are too many variables to determine how long a motor will last but lets just say you want to keep the BRZ for 10 years. If you don't mess with the motor it should last all those 10 years. It will likely last longer than 10 years even! I know some WRX that are over 300,000 miles and still run and dyno good numbers. Just depends how you drive and your maintenance schedule.
If you turbocharge your motor and it cuts the life in half it might still be worth it if you only plan to have the car for 5 years. I'm from the Honda world so I will bring up two motors. The K20 in the civic/rsx and the F22 in the Honda S2000. I know a few guys who have boosted these two motors to 500whp with mileage around 100k miles and have had their cars for over 3 years. There are some cases of the F22 being boosted to 800whp on the stock block and ran down the quarter mile over a hundred times. Is it practical? Nope.. But it survives!
THIS THING JUST WON'T BLOW UP!
Not all motors are built with flowers and marshmallows. Toyota is very well known to overengineer their motors. Some of their V8s last forever no matter how much you abuse it.