As context, I'm a middle-aged guy, 6ft, 180 lbs - a tough to fit "between size" for many ww lines. I'm a novice at whitewater, focussed on river running on Class II-III water. Other boats I've used include Dagger RPM and GTX.
The Supersonic was a perfect size for me and exactly what I was looking for --a great learning platform -forgiving, stable, yet easy to roll. (My roll has improved an order of magnitude in just a few sessions in this boat.) At the same time, I don't expect to outgrow this boat as my skills improve. I talked to expert paddlers at four different kayak schools who teach in the Sonics and use it on their own time as a creeker/big water ride. Overall,it compares very closely with the Dagger GT series. The GTs carve better and have more play potential ( sharper edges - full planing hull.) The Sonics seem to have greater hull speed, maybe a bit more rocker, more stable crossing eddy lines and are easier to roll.
The biggest negative for the Supersonic compared to the GT is outfitting. Presumably to keep costs low, Percpetion only offers this model with thier old-fashioned outfitting. Fixed thighbarce, glue-in/ grind-down foam hip pads, foam backrest and footpegs (which I actually like -- can stretch my legs out on the flat sections.) One strange thing is the total silence surrounding this boat -- you can find dozens of on-line reviews of the GT line, but none on the Sonics. I can't be the only guy who bought one! Chime in, folks!