Dear Jay,
The FAST model inherently includes many nonlinear terms and one cannot expect that a simple sinusoidal input to a nonlinear model will result in a simple sinusoidal response. Instead of computing an ROA by looking at the ranges of a response to various regular-wave (sinusoidal) inputs, I suggest instead to compute RAOs using white-noise wave input and post-processing via frequency-response functions. The process has been discussed in the following forum topic: RAO calculation - #4 by Jason.Jonkman (especially our ISOPE 2013 paper referenced in that topic: nrel.gov/docs/fy13osti/58098.pdf).
Best regards,