Yes and Yes.
When the Wizarding World of Harry Potter opened at Universal, a lot of people came to Orlando to see it. While they were here, they went to WDW as well, primarily to Magic Kingdom although some to Epcot also. The opening of HP coincides with when we began seeing hordes of "Thing 1" and "Thing 2" t-shirts all over Disney property, lol.
When the Diagon Alley expansion opens, a veritable TON of people are going to come see it. (Us included, in September, although we won't be visiting Disney property then.) And some of those people will either split stay at Disney resorts or they will buy tickets and come to Disney parks as offsite guests.
A rising tide lifts all boats. SeaWorld also saw a rise in attendance after the Harry Potter expansion opened at Universal.
Disney has actually profited nicely from Universal's Harry Potter work, and will continue to do so. If Disney ever opens anything as big as Harry Potter (and IMHO the only comparable thing would be Star Wars, Avatar doesn't have that big or devoted a following), Universal and SeaWorld will benefit from there being more tourists in Orlando.