according to Corkey Bell (author of maximum boost)
"Certainly a turbo will affect engine wear...If you drive vigorously but with some respect for the equipment, you can expect about 90% of normal engine life."
Basically the more power you have the more wear and tear you put on the motor. With a turbo setup you most likely wont be in full boost at all times. A turbo is only going to give you full boost under heavy acceleration. For example My GSR revs at about 4000 rpm at 80 mph with no turbo. If i had turbo im pretty sure that i could still go 80mph at 4000rpm without reaching full boost. So a turbo isnt always putting very much wear on your motor, only when you are driving hard.
(corect me if im wrong i havnt quite finished "Maximum Boost" yet )