having things setup for mode 3 like that, you're essentially working with an OBD2 style PID request string, which has its significant overhead drawbacks. sending a request for 6 bytes is going to take something like 16 bytes sent, then receiving 10.

I don't remember how much free PROM area there is, but I'm sure there is probably a huge portion of the 6811's internal EEPROM left open... perhaps modifying the mode 1 command code to reference a table in the EEPROM(which is able to be changed without a full flash) which could send out smaller(than mode1), predefined packets, which could remove something like 11 of the sent bytes for a packet request. could either keep those bytes off and have a lean response packet or use the now-freed space to throw in a few more values?