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.
1999 F250 7.3 Powerstroke "Fat Bastard"- SOLD
2000 Silverado Z71- Nothing special, now my sons truck
2005 Expedition Eddie Bauer- the wifes daily family hauler
2008 F250 CCSB 4x4 with a 6.4 Powerstroke- a few mods that deliver over 640rwhp (yes it has been dyno'd and been to the track to confirm the power)