to have realistic times for evry of the Le Mans cars (LMP, GT1, GT2), you have to set the AI at 120% strenght and the "AIDryGrip = " value in the track's gdb file set at 1.12.
R10, 908 and Pescarolo with the JPBS tyres will do insane times (at least, depends on how did you set the drivers' .rcd files), and the other cars in all the classes will do realistic times.
If you give them the SCC tyres, 908 and R10 will run around 3:20/3:22 and the Pescarolo around 3:24/3:26, which are more or less the times made for qualy and race (I'm talking about 2008/2009 seasons).