As much as I despise the way that AJ runs the GTP Endurance Series, I have to give him and it's participants full credit for making a Balance of Performance table that seems to work extremely well. It might give the OP a good idea on how to tune certain cars to make them run realistic and balanced lap times for their category, as long as you're good at driving on the limit and squeezing the full potential out of the car;
https://dl.dropboxusercontent.com/u...ULATIONS - 2015 GTPLANET ENDURANCE SERIES.pdf
In terms of real life LMP cars, the 2004 and 2005 versions of the Pescarolo C60, the 2005 version of the Audi R8, and the Audi R10 TDI, Peugeot 908 HDI, Audi R18 TDI, Toyota TS030 Hybrid, and Nissan GT-R LM Nismo were officially classed as LM P1 cars. Out of these, the Audi R10, Peugeot 908 and Nissan GT-R LM are your best bets when it comes to setting realistic LM P1 standard lap times in game, unless you're trying to replicate the uber quick lap times of 2015, in which case the Pescarolo C60 might be your best bet. I'm not so sure about the 05' Audi R8's performance level but it's probably a bit quicker than the Audi R10 and slower than the Pescarolo.
Meanwhile, the Audi R18 needs a bigger turbo to be able to compete at an LM P1 standard, whilst the Toyota TS030 unfortunately cannot be given a performance boost, and is slightly off the pace despite having very good handling and traction.
The Nissan Zytek Z11SN is the only 'true' LM P2 car in the game, but fortunately it does seem to run realistic lap times for it's class in stock trim. As you might have seen in the above BoP table, the Pescorolo's can be turned into pseudo LM P2 cars, and you could probably do the same with other LMP cars, especially the Audi R18 which is not that much faster than an LM P2 car in stock trim...
There are also many other Le Mans cars in the game that weren't officially LM P1 cars (the category in it's modern form didn't exist until 2004), spanning all the way back to the Group C era. Many of these cars can set LM P1 standard or better lap times in stock trim. LM P2 standard cars however are few and far between, unless you go out of your way to down tune a car to LM P2 standard.
Hope this helps you out.