Haunting past
Celestial energy was undoubtedly a powerful tool. At least in a way to power dedicated followers as the energy amplified their strength hugely. However, there were a few differences between when Mitchell transformed and when Peter transformed.
For one, the level of strength; even when Quinn gave the same amount of celestial points as he did Peter, Mitchell didn't have the same strength as Peter.
This was tested later on after Mithcell had some time to rest. The two of them fought, and there was a clear winner. It looked like the base strength of the person still had a significant impact as well. That also brought up another matter.
Mitchell also needed time to recover after transforming, which Peter did not, even if Mitchell wasn't hurt in battle. Just the change in his body seemed to cause pain all over. It was hard to tell whether this was due to Peter being a Wight or because Peter's body was able to hold more energy.