Sportlines suck there not for handling. They are more of a spring for the lowered look rather than the appropriate spring rate/height for handling.
The G2 springs designed by LG Motorsports are probably the best out of all the lowering springs out there for a 4th gen Fbody. As far as shocks for handling the custom revalved bilsteins that sam strano sells are probably best, followed by the koni yellows and then the bilstein slp kit.
What are you going for the same deal street/strip?