so far so good/correct
Your numbers are maybe a bit high, and I don´t think that the RB6 will produce twice as much downforce as the McLaren, but you general way of thinking is correct.
Now, there is another way to achieve this, which we should keep in mind.
You can stop the car going down, by using a progressive spring (e.g. spring plus bumpstop f.e.), or just a stop onto which the car will settle/rest.
In practice, they (RBR) will use, most likely ,a stiff(er) third spring/damper element to support the car at high downforce levels.
This will still enable them to run relative soft springs (at each wheel) for good traction/mechanical grip in low speed cornes, while supporting the higher (aero)load at higher speeds.
It´s unlikely, that they will run a higher ride height for this purpose, as it will reduce the overall downforce the car produces from it´s floor/underbody.