Sounds about right. I have seen anywhere from 320 to 346 from stock trucks. The 346 was a regular cab pipeline company truck that was actually a beta tester for the 6.7 six months prior to the release of the 2011 trucks and 6.7 engine. The driver told me the truck visited the dealer once a month and had been reflashed multiple times. 15-17% driveline loss seems legit.