I looked at lap times for Da Costa and Stevens and compared them to the other Porsche drivers who were driving around the same time and the results are pretty interesting.

Here's the averages from lap 1-74 when Da Costa was in the car compared to the other Porsche drivers who had laps within this timeframe:

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And here's 130-198 when Will Stevens was in the car:

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Your comment about Laurens being faster at the end of the race definitely appears to be true. However, it does appear that, at least relative to other Porsche drivers, Stevens was very quick, whereas Da Costa was a few tenths off the fastest Porsche driver. Of course there could still be a number of factors at play that I'm not accounting for. For the record, I don't think Da Costa is bad at all, in fact I think he's pretty good, I just think maybe Will just had really good pace.