When you don't have access to the company's internal figures, it's nothing more than opinion mixed with best-guess-timation. TEA/ERA can be wrong, off, or skewed by +/- one million in any direction. Anyone who accepts their results as the closest possible gospel are probably the same ones who use TP.com, or the like. Claiming to know ride wait times and calendar predictions based on previous dates is wrong on so many levels. Pass performance is not indicative of future results. Again, in an industry with millions of guests spending billions of dollars, either you're spot on or you're dead wrong. But if TP.com and TEA/ERA can make a business off people's gullibility, so be it.
That said, I'll play the game. According to the "industry," in 2009 IoA had 4.5 million attendees. In 2010, thanks to Harry Potter, that grew by 36%. Heck, let's spot 'em another 14% just to be nice guys. So with a 50% bump, IoA saw 6.75 million attendees in 2010.
Meanwhile, again, according to the "industry," DAK -- the least attended park in WDW -- had 9.1 million in 2009, and we can assume at least the same for 2010. So even with WWoHP, Universal still didn't come anywhere near Disney.
Here endeth the lesson.