[dekket]

For all the mentionings (throughout the series) of Tomas being diminished, there has never been any actually lessening of his power shown in any of the books.

At the end of A Darkness at Sethanon Macros (a notorious liar) tells Tomas to:

Seek inward, as well, Tomas, for I think you'll find that much of your power is now gone with those who were Ashen-Shugar's brethren. You still stand with the most powerful of mortals, but I wouldn't seek to master dragons, if I were you. I think they might give you a shock.

to which

Tomas said "I felt myself change . . . at the last." He had seemed subdued since his battle with Draken-Korin.

Macros then went on to say that Tomas retained a great deal of the Valheru heritage.

However, it there is only the word of Macros to go by. In the same conversation Macros also said that he

would move on to something new. Gathis will join me, and the others at my island are cared for, so I have no more duties here. I must move onward,

But obviously Gathis never joined him. And the entire conversation was merely to distract Tomas and Pug from the attempt of Macros to become Sarig (God of Magic).

Later, after Calis unweaves the Lifestone, Tomas looks different.

Gone was the alien edge to his appearance, what he judged the Valheru legacy. Now he looked mortal, a human male with elvish ears.

So after the Valheru are trapped in the Lifestone Tomas feels a bit different. And after the Lifestone is unmade he looks a bit different.
But we never actually see a difference in his powers/abilities. He could possibly still have his full array of Valheru might, and has only lost the connection to the (Valheru) Race, as the race no longer exists.

Anyway, we never knew the full extent of a Valheru's power anyway, only that they were far beyond the Eldar.

And as we don't know the full extent of Pug's power (who now is also likely far beyond the Eldar, and is even beyond that of Macros), it is difficult to decide who is the greater of the two.

Pug is however limited (despite his great ability) in that he casts his spells, whereas Tomas power is inate and instant.