Need to upgrade for upcoming comps

[Splbuck 0]

I have 1 crescendo 5500 @ 1 ohm on 2 18 sp4s box is 6.5 cub per 18 with 170 inch of square port! 18s are fully loaded. Pioneer deh80prs,4 rockford fosgate 5 1/4 and us 600 pushing them. 320 mech. Alt. 1 xs 3100 up front and 2 3400 in back of honda odyssey van. All 0 gauge cable. Never has it metered, but want more for comp. My question is this what about 1 more 18 with a wall and have amp wired to 1/2 ohm ( for comp) with some dynamat, or two more 18s? Either way will be just the one 5500 cresendo running subs. And how much diffrence do you think would be either way?