Sorry, it would be more sensational.
RvP > Benzema (hands down)
Fabregas > Alonso (different players, but still...)
Kaka (then) = Rooney (assuming Rooney is not "angry and confused")
Ronaldo (2009) v Ronaldo (2013), I don't know ho to call that one but it's safe to he say he's a better player now than then.
The real difference is that Real has been a traveling galactico circus over the last decade. This can't be saidf about United. Uniting RvP, Ronaldo, Fabregas and possibly Rooney would indeed be beyond anything Real ever accomplished over the last decade.
Whether this coming to pass is "plausible" is another matter.