I wouldnt go ABC box just cause of the rule of thumbness about it. As far as I know there isnt really math out there for you to go by at least the last time I checked. Im still thinking you should do a aero port box. Keep one set of ports that are tuned to the peak frequency of your car for when you compete and then another set for daily.
I sure as help hope you go up more than 1db from going to a pre fab to a custom box. I hate to nut hug but look up PWK designs they are 50 dollars. but he will use his software and take your given cabin geometry your subwoofers T/S parameters and then model different kinds of enclosures untill he finds you desired response. Which would be the loudest possible at your cars peak. might be worth it. Of course you can download the same software he is using but it will take you quite a while to figure out how to use it correcty