The drivers seem to be having a tough time with losing the rear grip they used to have. The blown diffuser probably allowed them to develop a "bad" habit of lifting off the throttle abruptly. With a blown diffuser you get more downforce when you lift off than in most cars, without it, you lose grip in the rear and have more weight transfer forward. So the blown diffusers allowed faster corner entry, which can often lead to faster corner exit, and by the end of the straight, even a faster straight line car probably won't catch up to you. Red Bull had the best one last year so they dominated tracks with lots of corners, especially high speed corners. In practice, without the rear grip off throttle, I saw plenty of times in practice folks were losing the back end in the braking zones.
Merc also has a system which is taking air from the rear wing when DRS is open and directing it down through a channel in the rear wing supports to blow it to the bottom part of the wing making for even faster straight line speed. It's been dubbed "Super DRS" by the SkyF1 commentators. So far Merc has highest straight line speed. The car is a lot better. But the boys didn't qualify as well as expected.
Adrian Newey has said that the entire car last year was built around the exhaust. That shows how important it was. So it's a big change for them this year. They have some funky winglets on the rear wheel hubs that the exhaust can blow onto. McLaren is actually blowing exhaust gas at the rear tires. Don't expect Red Bull to fall behind too much. RB are the Apple of motorsport. Slow, focused development. Incremental measurable improvements. Not just slap a bunch of stuff on and throw it on the track.
Hamilton seems to have his head back in the game. Vettel is excellent though, tends to keep his cool and be very mature out there, so that counts even if you aren't the fastest.
I can't wait to watch the race this afternoon. I can't watch it until 5pm eastern time due to other plans.
