I have the same situation, which only starts occurring right after the V1.06 update where they buffed the McLaren car.
It was right after the start on my season 2 MyTeam, where the McLaren was only the 4th best car on the grid, with quite some margin away from the Merc which is now technically the best. Lando ended up winning 4 out of 6 races (2 DNF), and always had like 20+ sec lead (once 30+ sec) at the end with Daniel in 2nd but was usually some distance away.
If I weren't mixing up the files, for post patch McLaren, other than the engine power tweaks (that every team had received and are identical), has a lighter and more balanced chassis, very slightly more drag but with quite a lot more downforce, but they also scaled down the upgrade accordingly so the values aren't really anything unusual comparing with other team.
But, for some reason, seems like the McLaren is missing <SplineElement x="1570.8" y="1" op="=" /> in the <AntiStallSuccessSpline> section which I believe every other team has it both pre & post patch (even the pre-patch McLaren had it but now missing). I'm not sure what it's for so I don't know if it does anything significant to the currently extremely OP McLaren.
Lando is definitely faster than Daniel ingame, this part is for sure.