the old 6.9 was a great little motor! it was underpowered and even with that turbo probably is not exceeding the limits of that motor. before i came in the army we had a mid 80's f350 with the 6.9 and it had over 700,000 miles on the original motor, never been rebuilt either.. it would start everyday, and had no issues except body rot. i say run it untill the truck falls completely apart around the motor.
2008 F250 CCSB 4x4 with a 6.4 Powerstroke- a few mods that delivers 659RWHP/ 1375 torque before nitrous