Well when the real life record at the ring is 6:11 recorded back in '83 by Stefan Bellof in the awesome 956, I must admit - it does make you wonder how realistic GT4 is when a time of 5:34 is recorded.
Also, don't let the fact the 1983 956 is an old car fool you. It dominated for more than ten years at Le mans and only started to feel dated in 1998.
Its probably the most successful (racing) sports car of all time.
p.s
Now that the 956's record at the 'ring' has being broken in GT4, the challenge now is to beat the 956's time at the unchicaned Le man circuit. In 1983, the Porsche 956 completed a lap in 3:16.5 - And down the mulsunne straight, the 956 was timed at 246mph.
So there's the challenge - to beat 246mph down the main straight and beat 3:16.5 around the unchicaned circuit.