Umm you think? Again go to LS!.com.au and ask for a guy with the nic PLan B. His LS1 ran a 12.2 with only an exhaust, small cam and LS1 edit. There are heaps of LS1s in the 12s actually, go look at the quick list on the same site, all with not much work at all. The fastest LS1 powered Holden is a 10 second LS1 in WA using a standard stroke 5.7litre with only a new manifold and cam plus exhaust and LS1 edit and a nice shot of nitrous.
All the older cars are much lighter, dont have all the good bits or safety and you can guarantee dont meet emissions or act as daily drivers (the commodores I mention do) and they also run on slicks, put slicks on the newer Holdens and they run awesome times, traction is the big enemy of Gen 3 Commodores. Remember again you are comparing factory cars which are not built with running 12s in mind!
Actually I read today that Plan B and and ALZ 57L are both running Flat 12s at the creek with 5.7litre cars but cant confirm this. Stupid uphill track! Anyway there are other LS1s running in the 11s allready at interstate tracks.