3-phase Motor RPM depends on power line frequency?
There are different designs of electric motor which can run on a 3-phase supply, and we don't know for sure which you have.  But given that the ratio of RPM to grid frequency is the same in both cases, yes, it is very likely that the speed will stay very similar at 340V.  This assumes that the load is low enough for the motor to run at the reduced voltage, of course.