The Sport Auto article provided by st-anger not long ago will shed some light on this. The article mentions problems in ALMS with the Murciélago, mainly due to the difference between the ACO regulations (ALMS and LM) and FIA regulations (FIA-GT). The rear wing is allowed to be much bigger with the FIA regulations and Reiter (who develops the R-GT for Lamborghini) acknowledged that they haven't been able to sort it out yet.
It's a shame Krohn-Barbour is giving up so soon. The only way to make a car competitive is to run it and keep improving. The 550 wasn't a winner straight away either.
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