M.86 at FL 280 results in a CAS of about 345kts and at FL 350 about 295kts. Since the airloads on the airframe depend on CAS it seems that prolonged or sustained operation at lower levels must involve a fatigue life penalty. Also the ride at 345 kts if turbulence is encountered must be interesting. The lower CAS at FL350 is closer to turbulence speed; if lower levels must be held and the likelihood of turbulence is there a lower speed may be prudent.