Since when is Farrar a cobble monster? He won the sprint of a big group for 3th place in Omloop, and a sprint of a big group for 5th place in Tour of Flanders.
That is absolutely not comparable to being the strongest on hills and cobbles.
For example, in pure cobble skill, I rate the ones who finished in the 4th place group of the E3 Prijs (Pozzato, Boom, Langeveld, Leukemans, Marcato) higher than Farrar... Because they actually made the race and escaped from the others on cobbles (except the train Cancellara/Boonen ofcourse)
Roubaix is a much harder race, in the sense of, there is no returning of big groups, the distance between each group and rider only becomes bigger and bigger. The only thing Farrar can do is hold on and hope to win the sprint for uhm... 14th place or something (and that WOULD be a class achievement already, I don't rate him that high)