LMP1-L probably has a bit more horsepower. From what I remember both IMSA DPi and ACO LMP2 would have around 600hp. I've let myself believe that the LMP1-L have a bit more. I could be wrong.
If the DPi continue to run with the spec Continentals, then they will probably be slower. I don't know any reason why the DPi would be faster than P2. The styling cues probably won't help the aerodynamics. Once again I could be wrong
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Basically I think that the new P2 will be very close to the LMP1-L. I mean, give the current P2s about 150hp more and they would be worrying the privateer LMP1s.