They used 1" to 1,:1/4" screws on mine and there was a high spot "titty" on my rear wall. I backed the screw up and put some washers on it, cause I didnt want it ( screw tip) to break thru the gel coat.
I see no reason to have over half an inch - 3/4" of screw going into the wall.
And dont use a self taper screw, drill it with something limiting the depth. After going thru the backer, reference that to a screw. U dont need more than a thread past the tip of the screw on the other side of the backer.
I'm afraid if you use a 2.5" lag, more than half will be sticking threw the wall.
Millertime- Sabre Handler