Very bad idea. I used the Alpine spec and used a 3" port on my 10" type R and it was noisy as fauk. I wound up cutting it out and switching it to a 4". Your going to need even more than that for a 15 or its going to be very noisy and very annoying.Well, this is what I'll do....
I'll build it like planned with the 3" port and if it sounds like a$$ I'll just seal the box up..... I just can't run that much porting in a box that fits my trunk nicely