I would be very surprised if Mclaren had the best aero this year, I'd say there was more chance(though very tiny chance) that the engine may have some mind blowing unmatchable feature/efficiency/power than them suddenly blowing past two teams who have brilliant aero packages. Ultimately they were miles off last year and aero is a series of changes and tests. Change the front wing, rear doesn't work as well, tune the rear, front needs changing, it's a series of steps where you hone in on something that works great.
Merc say aero/general ideas didn't change much between 2013/14, they started nailing aero in 2013 but needed to match that to suspension, understanding the tires, engines, brakes which they completed in 2014.
Mclaren will I think very much need to go through a series of improvements before they even come close to being in the same ballpark as Merc/RBR in aero terms.
But then the engine comes into it, if it overheats and they need to open it up more to improve reliability issues, they've got to start somewhat conservatively and I think significantly larger sidepods amongst other things point to that. Almost everyone at Merc will talk about how this team of people took 2-4 years to put together, was effectively in place for mid 2012, was building a car for 2013 and made a finished product in 2014. I don't think you can replace a guy midseason, bring in a new aero guy in September and have a competitive really anything by Feb/March, well feb/march 2016 is not a bad target, 2015, i'd be shocked if they did so.
As for the livery, I hope at the very least the red wing mirrors go, before testing if not the start of the season. I don't know, might look better out on track, the dark black around the rear of the car makes most of the detail barely visible, in sunlight it might have a bit more shape and feel to it. But black just seems, boring, well with very little in the way of other stripes of colour to highlight the contours in the area it's very dull.