In fact, given that McLaren have won the last 2 races, on merit, I would suggest that at present, McLaren have the fastest car.
Under performing: I think not.
The simple truth is that Alonso and Vettel are better drivers. You can't blame McLaren for that.
Two races a season doesn't make. Mclaren for YEARS have been uncompetitive for enough of the season that it has made a title all but impossible. There isn't any good being in the running for the second half of the season if they end up miles behind in the first half as has been the case for a few seasons essentially.
This year they've had a more competitive car much much earlier in the season, and threw up a crapload of points through bad pit stops, other problems and the black cloud over F1 that is Maldonado. Then they dropped the ball on development for large portion of the season and went from fastest at the start to 3rd and even 4th fastest at times for several races, which ultimately left both drives lagging WAY behind Ferrari a car that didn't at any stage have flat out pace but finished races, made the least errors and got the tactics right the most often.
Lewis to Merc though, can't see it, for money maybe. Remember if neither team is a viable option for winning the title and you can't get a seat at RB or Ferrari.... then 20mil to not win the title at Mclaren isn't a better option than 30million to also not win the title at Merc
However it all comes across purely as, his management took a meeting with them and Eddie put 2 + 2 together to get 94. He also mentions his team met with Ferrari, and probably spoke with Red Bull as well, some will be semi serious, some teams not burning bridges, take the meeting with Lewis's people to be polite because he might be the only or best option some other year. Also negotiating, more meetings they take with other teams, the better. He could leverage a fantastic offer from Merc into a better deal at Ferrari, which could push Red Bull into upping their offer, etc, etc.