That will be rather loud.
I am still amazed at the volume produced by my 10" sealed - small hatchbacks rule!
Give it a go without rear fill to begin with - all depends how often you have rear seat passengers. Although as they will be sitting closer to that 12" ported monster, they probably won't appreciate the stock rears anyway
SQ Plus get a good name over on the TA boards, so I'm sure they will do a champion job.
And remember, pics up asap!
Chris.