I see. If it's primarily for competition, even a 8" will suffice. First, competing in SQ competitions doesn't require you to crank up the volume and second, most "SQ" tracks doesn't have much information below 40hz really.

Correct. A subwoofer's performance is highly dependent on its enclosure. If it's a ported enclosure, it will also depend on its tuning frequency. For example, if you tune the port quite high then you won't hear the music below the tuned frequency - this is because the ported box's response dips drastically after that but if you port it too low then you aren't taking advantage of the free decibels the port freq gives you. This "fickleness" is why most people prefer to go for a sealed application for SQ. This is because a sealed box gives a very smooth response (if built right) and this is further complimented with the natural roll-off of a car's environment.

Alright, just PM me in Dec. I don't think my setup will change pa naman. I'll hype your expectations a bit since I offer one of the best bass-midbass transition among the cars I hear. It's something that I pay particular attention to ;)