The front bag is maxed out under the seats in the bow. I didn't see any extra space.
You probably could get more weight out of a enzo style sac when compared to the Supra SA Surf Sac
but not by much.
The rear locker of the SA measures 35" x 25"x 19" and the under seat area is around 50" x 24" x 10". The port and starboard storage areas are symmetrical. The Supra SA Enzo Style bag has a rear section that measures 34" x 23" x 19" and an arm that comes out to 30" x 23" x 9". You are probably going to have around 15" of space between the end of the leg and the divider in the under seat compartment. Possibly you will miss out on 100 lbs. At the end of the day it is not enough weight to justify the price of ordering a custom sac in my opinion.