…if [HCV] sales do get very large (which they will in the initial years), payers (even in the US) will step in and start controlling the costs (passing them more to patients, restricting treatment to F3/F4s, etc.
That would be a dangerous decision for the payers to make from a medical (and political) standpoint because existing risk-assessment tools are not accurate enough to say for certain that a given HCV patient can safely defer treatment. Such a reimbursement policy would inevitably lead to more liver transplants and cancer cases, possibly increasing the overall reimbursement cost of the patient pool in question.
Instead of restricting reimbursement as you suggested, I think the third-party payers will simply press hard on pricing, winning generous rebates relative to the list prices.