Looking at the live timetable on the Stena Line freight website this evening, it appears that an extra sailing has been added on Friday afternoon between Belfast and Birkenhead, returning Monday morning ex Birkenhead at 08:00. How this will work in practice remains to be seen though, as with an 08:00 departure from Birkenhead Stena Performer would not be able to operate the 15:30 from Belfast as she would not be back in time. More if/when I hear it (as there has been nothing official from Stena), but I assume some sort of rotation will be worked out with the Heysham ships (Stena Precision and the smaller Stena Hibernia).
Edit 18/10/17: Stena have officially announced these extra sailings which will add extra capacity in the run up to Christmas. At present it looks as if Stena Performer, Stena Precision, and Stena Hibernia will take it in turns to operate as the third ship on the Birkenhead service for a week at a time, with the other 2 vessels operating to Heysham