Is there a difference between SM231AE and SM231AD speed torque curves? (or in general, does the encoder affect the speed torque curve of an SM motor?)
The answer is YES, sometimes. The -E encoder is the 1000ppr encoder, and it is limited by its maximum update rate. Any SM motor with the -E encoder cannot run over 6000rpm since the feedback information begins to degrade above 6000rpm. The -D encoder, on the other hand, can handle about twice that since it is a 500ppr encoder. Since none of the SM speed torque curves go up to this speed, you will not see the -D encoder limitation.
If you are looking at an SM speed torque curve that shows torques above 6000rpm, it must have the -D encoder to reach these speeds. If you are looking at one that stops at 6000rpm or before, then it is the exact same curve for both the -E and -D option in that motor size & winding type.