I don't have data for the Sonnet but cockpit location is determined by fore-and-aft buoyancy distribution which of course is determined by hull shape. There is no fundamental hull shape difference between strippers and SOF.

Many modern kayaks have the buoyancy shifted aft of midships with corresponding rearward location of the cockpit and the assymmetry is very noticable in both the plan and profile views. In a symmetrical design like my canoe the seat location places the CoG of a solo paddler more-or-less centrally although I have a few inches of adjustment to accommodate certain wave conditions and also leg length differences since it is only 12' long.

So look at the shape of the hull. Typically for a symmetrical hull the backrest is about a foot aft of center, i.e., 9 feet for a 16 footer. 1-1/2 feet further back is about 10% of length which is a bit extreme in my experience so the asymmetry would be very apparent. If it isn't check again very carefully and perhaps contact the designer to make sure you have understood the instructions.