There are several "Psykers" of different species (namely the Eldar) that might have even posed a threat to the Emperor. For instance he was working very hard on developing parts of the Eldar webway for Humanity. (Presumably the Eldar figured that stuff out before the Emperor's balls dropped.)
Not quite. The webway was created by the Old Ones and then inherited by the Eldar.
The problem the Emperor had was that he couldn't replicate the material used in the webway's creation to build an artificial bridge in order to 'hack in' to the established network from the Imperial Palace. Thus he had to use his own psychic powers (via the Golden Throne) to maintain an artificial bridge by which Imperial troops could then access the wider webway network. This cannot be used as evidence to suggest Eldar psykers rivalled the Emperor's power. In fact, although it is impossible to prove either way, it does seem that the Emperor was the most powerful psyker the galaxy had ever seen.
Also it is specifically stated that if Humanity had not expanded that the Earth would be prone to Xenos and/or Chaos influence.
I do not think the Emperor was the most powerful Psyker, he was certainly not as gifted in foresight as he thought he was. Blinded by love for a son...REALLY? Considering the way some of the Emperor's son's were treated it sounds like the Emperor just liked the first Primarch he found the best.
The Chaos Gods actively worked to hinder the Emperor's foresight in the build-up to the Heresy. Again, this can hardly be used as evidence to suggest he wasn't the most powerful psyker.