stock gsr with a greddy kit 5.5 psi expect low-mid 14s, and a stock gsr wtih a drag kit, expect really high 13s or really low 14s. thats with everything else remotely stock, and those times can be severely lowered with minor additions
people tend to be unrealistic with their car, and expect to slap on a turbo and be hittin' 12s, or a quick n/a set up and hit 13s.
and obviously it's safer to use higher boost in an LS(over a gsr)... but even though the compression ratio is higher in a gsr, that means it would need(theoreticaly) less boost to reach a certain hp figure. probably around 3-4 psi if everything is identical.