The cars don't handle the same when at the same level of wear, there's not that much of a difference though, it's more of a split.
The 908, at say 50% rears will start to get extremely touchy with the over steer. While the R10 gains under steer on turn in, and if you're careful on exit, you can get away with decent times for a while. When I tried the 908 for the same stint length (could be my inability to control a tail happy car versus the R10's strange handling on worn tires though) the times would be fast for about 5 laps then just come crashing down immediately after that like a second a lap. While in the R10 I can do 2:11's from about lap 4 to 8 then I start getting into the 12s and 13s, but by then I'm done with a good stint.