Easy solution. Jon Sisk went through this “drill” for his RV, about a year or more ago. GAMI gave him a letter authorizing him to reference GAMI’s approved data in support of the change (it is major, by regulatory definition) to his stated operating limitations. Not sure, but I may have also signed an 8110-3 for that change that went along with the letter. He took it to his FSDO person with the proposed new operating limitations already prepared.
FSDO looked at it and said “yes”. Not an issue.
However, we have been asking (and asking ^10th power) the FAA to address this situation since September of 2021 ! ! ! Senior FAA management keeps promising to take action to “streamline” the STC and related approval processes.
BUT . . . so far nothing has happened.
Very frustrating!
George