It all depends indeed on how hard you drive.
When driving normally (well, that's semi-hard driving in a standard GT I think) you are hardly boosting, maybe 1 or 2 psi. This is hardly any extra stress to the engine.
Since you have more power in lower rpm's you can drive the car at lower rpm's and still have fun instead of driving a standard GT on higher rpm's.
I am not saying the turbo will lengthen the life of your engine, nor am I saying it will be just as long. But stick to a nice 6 psi and personally I don't think it will matter that much.
I drive about 10.000 km (a little over 6000 miles) a year and I hardly care if my engine dies after 250000 km, that's another 20 years for me...
even at 200.000 I don't care.
However since the car will be much faster you will probably be tempted more to drive it harder than a standard GT and then it's a whole different story.