First off, listen to this David guy...he is hands down either the smartest dude in the sound business or the worlds best bull@$%!er...but
I'm going with the first cause he has helped me out in previous posts, ha!
Here is what I can add to the conversation: 2007 X Star stock factory (4) towers running off a 600/4 and I have a single 13W7 in a ported box run by a 750/1. It sounds awesome and slams hard...on Torch lake this summer the 13W7 would draw people from across the very crowded sand bar to see where the hell all the base was coming from...it is all you will ever need. Makes an already great factory JL system seem anemic.
Now David did recommend that going to the 1200/1 will make the thing come alive vs. the 750/1. You will be very pleased if you go with the W7. I plan on taking the box from my current boat and putting it in a '14 X Star, then upgrading power to have 200W running to each of the in boat and using my 750/1 on the towers. There are level controls for each of the zones. I'm hoping it doesn't suck...can't imagine it will.