Most defantly a 5 minute time could be had by someone in a very high powered, high downfroce car. I.E, LMP's F1's, GP2's........ect Someone really could dip that low, but really none would wan't to go flat out at the ring just for the reason that its so dangersous going THAT fast, especially if your not on a closed track where theres no traffic. With little bikes, slow sedans and wagon's on the track you would never be able to really give it your all because around every bend disaster could happen. When was the last time that Lemans series wen't to the ring, 82 or 83? Wasn't a 6'1x.xxx time done? 20 something years later I think better can be had with today's technology.
But really in GT4 it all boils down to tires, the tires, those dang R1,2,3,4,5 tires and GT's fact that you can really max out a car big time in every aspect to go faster making actual real life time comparisons much like hitting the broad side of a barn with a rock in the middle of a blizzard after hypothermia has set in and you can no longer feel, anything. Race cars should have S tires and street cars should have N tires and not modding your car for more performance. Thats your best bet for getting a semi accurate comparison.
Perfect case is the RUF CTR 87' Yellowbird, get one dead stock in GT4 and slap on a set of N1 tires, turn off the panzy driver aids and wallah you'll be going nearly the same speeds Stefan Rosler did when your giving 100% and getting 100% out of the car just like in the Fanzanation video when he wasn't in traffic in his RUF CTR 87' Yellowbird that was showroom stock.