My Boeing FCOM for the 757 cites the procedure you saw in the sim:
"Runup to a minimum of 60% N1 for approximately 10 seconds duration and confirm stable engine operation before the start of the takeoff roll."
Although the notion of blade twist is interesting, it troubles me. The fan blade angle of incidence needs to be pretty rigid in order to predict performance; I'd be surprised if there was much change in the angle with RPM. The blades will seat in their channels rather smartly upon acceleration. But other than a certain degree of creep (elongation), I believe the geometry needs to be stable. So that leaves us with centrifugal force.
