Step 1: Internally determine if the performance claims by owners are accurate or not. That step already appears partly in process for the Danish group.
Step 2: Fix any firmware/car issues that may be limiting performance.
Step 3: Put together an information package explaining how performance and related numbers were reached so that current and future customers are educated.
Step 4: If any performance and related numbers were erroneously arrived at - person/s responsible are fired.
Step 5: Contact each letter signer individually, have them sign an NDA/can't talk publically etc., etc... then explain what happened from Step 1 thru Step 4.
Step 6: At this point there can be several outcomes depending on what was discovered through prior steps and how each letter signer feels about that.
While I agree (but the fired part, if you fire everybody every time somebody makes a mistake people will never learn), I would add that once step 5 and 6 is done, move back to honest advertising.