I personally think that Chevy should use the "SS" as the performance package across the line.....similar to AMG (Mercedes), M (BMW), V (Cadillac), etc. Probably closer to SRT (Dodge) and SVT (Ford), but still.....
Cobalt SS....absolutely....needs a minimum of 220 HP and a good looking SS-specific body kit (no big-ass wing on the back...save that for the aftermarket)....this would at least give the "ricers" an American option...compete with the Civic Si, the Sentra Spec-R, the GTI, etc.
Impala SS....again absolutely...Chevy has done this one right to an extent....good power (more would be better, but....)....needs better looking wheels and some body differentiation (front/rear fascia change, maybe some rocker covers)...compete with the Camry, Accord, "Taurus", etc.
Same idea across the line....
Equinox SS...needs close to 300HP and a seriously upgraded interior.
Trailblazer replacement should put out 320-350HP
Tahoe SS...350HP minimum, AWD from the Escalade
Silverado SS...400+HP motor and AWD used in Escalade/Sierra Denali
Probably don't bother with an Aveo SS, but pretty much everything else should have an SS option....it just needs to be a real upgrade, not a plastic body kit and big rear wing only package.