I am still unclear why one person should think Grazz't to be any greater or weaker in power than any other demon lord or devil prince.
In all the official stuff from years gone by, Grazz't was not as powerful as Demogorgon, and certainly was about as powerful as Fraz-Urb Luu (that demon-baboon prince)

Grazz't was not written as insanely physically powerful; Grazz't was instead a prince of deception.
Who were the insanely powerful ones? Orcus and Demogorgon. These two were to each other what Asmodeus and Mephistopheles were to each other - the head Bottom-Feeders in the Pond of Evil.
The use of one product's rules in another is a different discussion entirely - suffice it to say that WotC's official stance is to NEVER assume that a gamer has one certain product, and do not make content from another book dependent on content the gamer may not have. I for one prefer this policy, though I understand others do not. In my opinion, it is far better to add Epic levels or Divine ranks to the demons, than to assume that all gamers want it that way.
This is a major sticking point:
WotC's and Monte Cook's vision is one of many; there are other ways to portray the Demon Princes and Devils, and not all people will be satisfied with the vision that WotC presents.
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On a side note:
In fact, not to offend Greymarch or any others in any way, but I think it would be a really good discussion topic, to discuss why some gaming groups demand official stats from the parent company, while some groups do not. Such a discussion would also tie in with why some gamers feel that some products are unnecessary, such as rules for vile actions, rules for naval and mass combat, rules for playing out diplomacy, etc. What's official, and why do we care? (or not care?)
After all, this topic of "official rules" sometimes splits the gaming community right down the middle! An honest yet peaceable discussion might be very helpful in understanding.