Triradial makes little sense on a sail as small as a SJ-21 working jib especially when working with woven polyester fabrics. While tri-radial cutting makes a lot theoretical sense, the reality is that much harder to get the fiber orientation, stitch tension and broad seaming sufficiently accurate that a small triradial sail ends up with a better flying shape than a simple broad seamed cross cut sail. I personally spent the money to go to the tri-radial route on a small high aspect ratio jib, and ultimately went back to a broad seamed jib to replace it for performance reasons.
Beyond that, tri-radial sails produce a lot more fabric waste, require a lot more skill to build well, and are far more labor intensive to build. Now then if you want a real improvement in performance I would consider adding a full length head batten and slighlt over length lower battens, then building the sail with nuetral or even positive roach.