Regarding your 27% vs 28% attach figure, note that a far more significant part of the PS3's install base is located in Japan (~16% vs ~3% for the 360). GTA IV hasn't even been released there...
Far too early to say, after christmas we'll have a far more complete picture and even then the game will continue to sell I think there will still be sales potential when a cheaper slimline PS3 arrives. In any case a simultaneous release was expected to help GTA IV sales on 360, just like a simultaneous release of Resident Evil 5 will considerably help PS3 sales.