I think so much of how you evaluate this depends on your definition of what "star power" is too. It's a bit of a fuzzy term and as both of you have pointed it, a term that's usage is not really consistently used through time. Given the difference in media environments, it's hard to make an objective comparison, apples to oranges if you will.
I always think of this same thing with Madonna. To me at least, I'd pinpoint her absolute peak in terms of cultural relevance in America to the Blond Ambition Tour. For lack of a more objective term, that seems to be the time she mattered the most here. It's kinda hard to compare a time like that to something like Ray of Light, which was a huge hit in it's own right here and she was certainly on her share of media outlets and was back in the public eye and made lots of money and garnered critical respect, but despite all that, she had significantly less relevance in the mainstream culture. To me, that's a much more significant measure of star power -- in terms of how much influence you have over the industry and the cultural environment.
It would be interesting to see Mariah's Q Score since 1990, which is both a measure of popularity and likability among those paying attention to her.
Edit: One more thing I wanted to add, as I was thinking about this more in the shower :p
I think that stuff like the perfumes, the endorsement deals, etc. definitely add to her star presence, but I'm not so sure I am convinced they add to her star power, because she's not a dominant force in any one of those aspects. Her influence in 2005 (compared to 1996) may have been a mile wide, but it's an inch deep.
Around Daydream, she was pretty much without contention the biggest musical force in America. She was riding a huge crest that was about to peak, with a long string of hit albums and singles. The whole thing was very meteoric, but with TEOM, while it was a hit "comeback" album -- it looks to have been a brief uptick in popularity, but not overall influence.
Like I said, it really comes down to definitions and I can see the argument being made either way. That's my 2 cents for what it's worth (about 2 cents!) 