The RB6 is certainly the car that sets the pace. It used to be that the Red Bull was strong in fast corners because they had high downforce and relatively good aerodynamic efficiency. This was much owed to their superior front end design that gathered much air and build up a high flow under the floor. They did not have good traction in low speed corners though due to a compromised pull rod rear suspension and gear box layout.
This year they have kept their advantage in high speed corners although the downforce is much increased with the 2nd generation DDDs and have added good traction. I guess they still had the best front end design and added some refinements found by their competitors. But crucially they sorted the back end for good diffusor flow and optimal traction out of slow corners.
The RB6 with the pull rod suspension is incredibly narrow compared to other cars. I guess last year they never got a good compromise for aero packaging and grip. This year they have made jump forward, although according to Scarbs working the suspension is a nightmare for the mechanics. Any change to the damping requires a gearbox/floor job.
Their only "weakness" may be potential top speed on straights. Without effing duct they are giving some top speed away in the dry or downforce in the wet. If they get that they may shave another 0.5 s off their lap time in Barcelona.
So how is that comparing to W01? Probably the car is not good enough in terms of front end air flow which hurts the downforce and may have fallen back on traction as well. Merc have lost four races to get in a shape that they should have had in February and now their potentially stronger driver is 50 points behind the plan. They are fighting two handicaps this season. They have another aero upgrade coming in Turkey after Monaco. If that doesn't move them ahead of Ferrari and potentially McLaren they might as well focus on next years car and hope that the front end design will radically change due to regulations. The way it is now, Newey is making all the points there.