Talking about the Lambo is un poisson rouge because there are no GT1 cars which can do 2 hours without stopping. The Lambo will always have a fuel consumption disadvantage and forcing an extra stop won't make any difference to that.
Lister can do 60 minutes on one tank of fuel, well it certainly could in GTR1 so it would only need 1 stop giving it a big advantage.
In GT2, if the BMW can't do an hour without stopping that's because it's totally out of place in the class anyway. I'll wager nobody has bothered trying to set it up for a long endurance run with low wing and longer gears and frankly is't so much heavier than the other GT2 cars (regardless of ballast) that it can't expect to be competetive in anything longer than a sprint race.
I raced the Endurance stuff last season in BMW and yes I agree it's a bit out of it's class being in GT2 (hence the big weight penalty), but not that out of class. It can do 60 minutes on one tank so I guess it's not that big a problem in GT2 like it is in GT where some of the cars can do 60 minutes and some even on full lean can not and some can at a stretch do it especially if the new classes me that M3 is lighter and therefore slightly better on fuel.
Don't know about the Mosler, but it's been used so little I don't know if anyone has any genuine data on it when setup up for endurance racing.
I raced the Mosler in GTR1 in Endurance and the fuel consumption is no different in GTR2. It's as bad and probably worse than the M3. Again probably could do 60 minutes on one tank, so probably not a big problem compared to the GT cars and again if it's moved to a different class for next season probably not an issue. The issue is really in the GT class.