A final decision would be expected only after ALL appeals have been exhausted. If Sarepta (or Biomarin) is dissatisfied with the PTAB decision they could take it next to the Federal Circuit Court of Appeals. That would be a lengthy process and etep (if approved) would almost certainly be on the market by then.
It could be inconsequential or it could not be. If Prosensa's patents are found to be blocking in the U.S. and Europe, the optimistic scenario for SRPT is that etep is approved, drisa is not approved, and Sarepta will likely have to pay Biomarin a single digit royalty. What if, however, both drugs are approved and Biomarin moves to block the sale of etep based on infringement of their patent claims. That could be just a bit messier.
The issue is not whether or not Sarepta's composition of matter claims are valid. The issue is whether or not the Prosensa claims covering etep are valid and upheld (as they have been so far in Europe).