Lost steps are typically the result of trying to run the motors faster than they are capable of. The simplest answer is to reduce the velocity by 25%, and possibly the acceleration as well.
No, you can't add a 10V power supply. But bumping up 8-10V higher probably won't make a lot of difference.
What are the full specs of your motors, or the model #? You want motors with an inductance of 3mH or less.
Going to larger motors will likely result in more lost steps. The bigger the motor, the faster it loses torque, as rpm's increase. So a larger motor may have less torque at higher rpms than the ones you already have.