Sorry, Kappa was made for 2-seater roadsters. The Silvia (and Prelude, Celica, etc.) has always been a 2+2 coupe...Kappa, kappa, kappa!!!
Even if GM could muster some sort of fixed hardtop to a Kappa 2+2 vehicle, its trunk space would probably still blow compared to a true coupe's.
GM could probably use Alpha, but there might be cost and size issues.