I understand that encoders are important in robotics, but Firmata is not limited to robotics applications.  Currently the midi message format is only used for features that are part of a microcontroller (analog and digital pin values, pwm values, etc). This is why adding a feature peripheral feature (rotary encoder) to this part of the protocol is actually a big stretch in terms of the current protocol. If microcontrollers had built in rotary encoder support on all or certain pins this would be a non issue.

If you go with the sysex version I drafted above, I can accept you pull reqest as soon as I test with a Leonardo. If you want to keep the message as you have drafted it, I'll need consensus from the larger Firmata community (there are people who have created client applications in many different languages). To get that process started I suggest you send a message to the firmata devel list: https://lists.sourceforge.net/lists/listinfo/firmata-devel to explain your case. My job here is to make sure that changes introduced are in the best interest of Firmata users and developers.

One other thing you can do to optimize is increase the baud rate to 115200. There is nothing preventing users from increasing the rate.