Xanthe did say that in order to use other monsters' abilities instead of just their body parts he would need to transform his own energy or life force, which would weaken him. I think what he meant by that was that his ability to transform is a result of the energy currently flowing through his veins, but that energy is also responsible for his regeneration, it's not like he has two different sources of power for his Memory and Body abilities. However, to use other abilities he would either need to spend some of his energy to transform the energy to his chosen monster or superhuman, and if that transformation is permanent, he would lose some power permanently for his regeneration, shape-shifting and memory abilities. What this means is that his total amount of power would not decrease, but it would be divided and the monster energy he can use for his shapeshifting cannot be used for his other abilities. As a result, he would need more power in total to have the same amount of monster energy available for shapeshifting before his transformation. For example, let's say the Mc has 600 monster power, he transforms 100 of that power to Valravn's darkness. The Mc still has 600 power, but he can only use his 500 power for shape shifting before he runs out. As a result, the MC's shape shifting powers have weakened despite his total monster power pool not changing at all.
However, if the ability to transform his energy and use other abilities is temporary like the Silats, then he will be burning through far more power than before, because he will need to constantly spend energy on it like his other transformations, and his efficiency will take a nosedive. Unlike Michael, the Mc doesn't have any rapid direct methods of recovering his energy, his digestion might be fast, but it wasn't instant in the Cole fight, when Deryl fed him.