Tahu is confirmed to be way more powerful now than he was in his Nuva days. In fact he is considered the most powerful Toa to ever exist in his current state. His OVERALL power is greater than Tuyet with the Nui Stone, but elementally speaking he is less powerful than the other Nuva.
There are two things contributing to this amazing power:
-
Tahu has all 42 kraata powers - this is of course not explicitly confirmed, but the opposite is statistically impossible. There were tens of thousands of Rahkshi in the final battle and we know there were many different colors and while yellow (heat vision) was predominant, it was not exclusive. It also wouldn’t make tactical sense for Makuta to not send some of the Rahkshi types into battle.
-
Even disregarding the abilities granted to Tahu, he still absorbed the energy of an entire army of Rahkshi. In this case, we are talking about physical energy, not elemental energy. When we realize a single level 6 kraata is comparable to a Toa, this becomes a tremendous boost. It is even more impressive because we know from previous story (2006) that boosts in physical energy increase the power of all abilities that rely on it (in this case kraata powers).
So essentially, current Tahu is Makuta on steroids, but with elemental fire power of only a regular Toa.
Arguably, his Stars form IS his desert form.