I threw out the battery blanket when I got rid of the Optima. Got something bigger (1000 ca / 850 cca) from WallyWorld for $75. As for the warranty, I find it difficult to believe that they would be restricting the warranty to 'approved' fitments, but then again... Advance has been suffering a lot since they bought Discount Auto. Discount was a MUCH better company.
If you are concerned about the warranty stuff, Wallyworld or Costco are the best bet. Wallyworld you need to save the thermal paper 'date verification' receipt... Which they "helpfully" suggest to slip into the pouch on the battery, so it is in the presence of the nice hot engine... B/C Thermal paper wouldn't ever turn all black when faced with a lot of thermal heat.
Costco just saves the purchase date in your account history that they access when you go back in. I've never gotten less than a full refund on the batteries from them.