The Wilson Security Porsche Cup S of David Wall/Barton Mawer/Greg Crick also remains in contention for victory, sitting one lap down in fourth place.
While having a distinct speed advantage, the Team Joest and Team VIP entries must complete three more compulsory pitstops than their pursuers, courtesy of running 2010-spec GT3 machinery.
The first six hours of the race have seen competitors challenged by intermittent sprinkles of rain, with Luff starring in the slippery conditions during a double stint.
The Roger Lago/Matt Kingsley/David Russell Porsche leads Class B in fifth outright, despite a trip into the sand trap at Hell Corner in Hour 3.
The Theo Koundouris/James Koundouris/Steve Owen Class A Porsche sits sixth, some nine laps adrift of the race lead.
The two Ferraris, crewed by Hector Lester/Luke Searle/Allan Simonsen and Peter Edwards/John Bowe/Tim Leahey are seventh and eighth respectively, with the latter entry having suffered a loose door and an electrical problem in Hour 2.
The David Reynolds/Dean Fiore/Marc Cini Porsche runs ninth after suffering steering damage in a collision with the Mal Rose/Tony Alford/Peter Leemhuis Nissan GT-R at The Dipper while running fifth. The Donut King Nissan had earlier lost time with a gearbox issue.
The Nathan Tinkler/Jeremy Gray/Steven Johnson FPV suffered delays thanks to an off at Murray's Corner.
The race's first casualty was Dean Grant in the Procon Developments Mosler – crashing out at the end of Conrod Straight on Lap 12.
Class leaders after Hour 6
|Audi R8 LMS
|Porsche 997 GT3 Cup
|Lotus Exige S
|Mitsubishi Lancer Evo X
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