LMP2 was from my point of view quiete disappointing in the last years, just cruising around and finishing was sometimes even enough for a podium.
I think the battle between the Chevys, the Astons and the Ferraris will lead to a high overall pace in that class, I expect the quickest GT1 to finish ahead of the quickest LMP2. Even the big surprise is possible, remember 1995 when GT outran the prototypes, but, OK that's a remote chance this year.
It's also not only about cars, but about the drivers as well. Looking at the drivers roster of teams like Chevrolet and Aston Martin, I didn't find a match for that in whole LMP2.
Not sure, but I think GT1 has 10 litres more of fuel in the tank(100 compared to 90 in LMP2). But, of course, I guess the 1100 kg 600 hp Chevy will need a little more of that than a 3.4 litre Judd in a 750 kg car also...