The bottleneck is whichever process is slowing everything down (as I explain in my free lectures).
At the moment the senior stylists are the bottleneck because it is them who are limiting the number of both cuts and treatments that can be done (the assistants and junior stylists could serve more customers but they are limited to the number that the senior stylists can deal with).
If they employ another senior stylists then they could do a lot more cuts and treatments.
They would still be the bottleneck for treatments (because they would still be able to deal with fewer customers than the others). However the bottleneck for cuts would be junior stylists because the would only be able to deal with 8,000 customers whereas the other two would be able to deal with more.