1) The SM motors are rated for 7500rpm since the mechanical balancing of the shaft is performed at that speed. A motor can be balanced at higher speeds, it would just need to be requested as a custom product for that speed. This is also true for the NEOMETRIC motors.
2) The SM and NEOMETRIC motor flyers state the following about that the speed/torque curves are "with 500 ppr encoders. For 1000 ppr encoders, derate to 6000 rpm." This due to the output frequency limitations of the encoder on these motors. The encoder is rated at 100 kHz max output frequency. For a 1000 line encoder 100,000/1,000 = 100rps = 6000rpm.