If you have the data from before and after, it might be worth contacting GM directly (not a dealership) about a buyback. I've read that GM started denying buybacks because the final fix restored 100%, but in your case, you can show that's not true.I've been collecting data for the past week since my update. Based on that data, my battery pack now only can charge to ~54 kWh. It was 60 kWh the day before the update. I'll continue to collect data, but everything points to the fix reducing the effective size of the battery (and consequent range) by 10% and relabeling it as 100%. It's not just ranting. The drop in range was immediate and very noticeable. My efficiency is irrelevant only the % of battery charge drop for the kWh expended. (My efficiency is exactly the same as it was before the fix 4.1 - 4.3 miles/kWh but the miles of range is down 10%.)
You have to contact GM directly - use the concierge number 833-EV-CHEVY. What GM offers (if anything) may depend on the lemon laws in your state.