This could potentially work in your favor. The only day lines were snaking around the park all the way to the movie theaters was the actual grand opening day, days after were really bad too, but just to get into Potter.
What's not usually mentioned is the days leading up to it (although I think media only stuff existed the day or two before grand opening) was the soft opening previews where they let *just* the onsite guests in first, and then maybe just a few regular guests that happened to be at the right place at the right time.
Two weeks before grand opening we hit one of those moments, and since they closed it up to guests pretty quickly it was actually not crowded at all in the preview.
I'd expect another huge line for Diagon Alley grand opening, particularly for Hogwarts Express journey from London to Hogsmeade. That will be a dream for many hard core Potter fans. It may not be as huge as the original Potter line, (or at least a lot more dispersed)
We did go to the potter grand opening too, but gave up trying to actually get into Hogsmeade. We did make it inside IOA though before they closed the gates. It looks like the people that actually made it in to the land, close enough to see the opening ceremony, before they started limiting the people got in around midnight to 1AM to wait. (Based on going up along the line asking people when they got there)
Universal's grand opening ceremonies are pretty impressive though, the Transformers opening was awesome.
So my guess with the perks you get from staying on site, the only day that could possibly be affected by it is the grand opening date itself. So if you have a chance to preview it, by all means do it then before it gets too crowded, and enjoy the rest of the park on the day of Grand Opening (IOA, although there may be a few taking the back trek on Hogwarts Express there)