Roar hasn't gained any increase in power apart from speed. But even making full use of his speed will take some time getting used to.
Any increase to raw power that Roar can have due to his advancement to the third mortal coil will take time to realize. That is the nature of the third mortal coil.
However, the things he gained, such as an awakened mind, the increase to intelligence that accompanies it, and the ability to sense mana, are all life-changing.
Now that Roar can sense mana and his intelligence is increasing, he will be able to cultivate techniques instead of relying on eating to grow stronger.
Roar is also be able to cast spells now. This is perhaps the greatest change in his capabilities. It is what both loose cultivators and the majority of cultivators from families will be jealous of.
So Roar has a lot of things to learn and many newfound capabilities to explore. Arthur can't blame him for being so enthusiastic about it.
