No I think that's a common misconception, the 2010 Peugeot 908 HDi FAP was the fastest prototype car in history. The modern cars have far more downforce & braking distance are shorter, this is where their making time. The group C had great straightline speed, but were 10-15 secs off the modern cars just in Qualifying at LeMans (spa being closer). In the race they were actually slower as they detuned for reliability, modern LMP1s can now pretty much run Q pace during the race...
EDIT, the fastest Group C was either the C11 from 1990 or the 89' Sauber, from what I can tell. You can check out Pole Laps from over the years:
http://en.wikipedia.org/wiki/1000_km_Spa