That list definitely needs Nightwish. Probably Blind Guardian too. But I would agree with you aside from that.
Yeah, I'd say in terms of drawing power, the top bands are:
Nightwish, Blind Guardian, Kamelot, Sonata Arctica, Dragonforce (even though they've become so popular and liked by other people it's not really fair to judge them on this scale), and to a lesser extent, Edguy and Hammerfall - though it's hard to really tell for how little Hammerfall comes here and how much Edguy does. I've seen Edguy four times, all the shows have had good turnouts. That's 4 times in 5 years, and if anything the crowds have been growing.
Gamma Ray is another band that I think actually has a decent chunk of drawing power, but it's hard to say because they so rarely come over. NYC in '06 on the Majestic tour was packed as I recall.
Oh yes, and though not truly power metal (though about as Power Metal as Nightwish is, to be fair), Symphony X has a good drawing power here as well (well, they are FROM here). Again, seen them 4 times and all the shows have been packed.
On a related note, Firewind's headlining off-date in Reading, PA in September '07 was one of the saddest things I've ever seen. There had to have been less than 50 people at the show. It was so bad that my friend and I stood outside until the final support band went on, and just walked in and stood front "row". It was basically just a loose cluster of people, most of which weren't even knowledgeable about the band's music. After the show the guys just came out and hung out with us since there were so few people there, it was really cool of them. I've just never felt so bad, because it was an absolutely fantastic performance, but all it wound up being was a testament to how amazing bands go overlooked.
Honestly Stratovarius/Pagan's Mind in Baltimore this past September was dismal. Again, I almost felt bad. Not as much for Pagan's Mind as Strato. PM got a fantastic response from the crowd that was there, but Strato performed to a sluggish, tired, somewhat unenthusiastic crowd of under 150 people.