Sorry to here you are having issues. Hopefully GM can get things resolved to your satisfaction.
On your "REDUCED ENGINE POWER" issue, it looks like there could be a TSB that might apply here (Service Bulletin Number : PIP-4673). Hard to tell because that one isn't very descriptive. You can check it here
Overall, f you don't feel that your issues are being resolved by the dealer then I would call the customer service number and explain the situation. Have them issue you a case number and due their investigation. Let me know what comes of that and we can go from there. If your concerns still aren't addressed to your satisfaction, then we can escalate per the process outlined above.