It won't be a huge dent in the capacity, plus Potter is a small space that can only hold a certain amount of people, thus most of the park will be people with Return Time Passes. That means Potter is essentially the only regulated area in the park, with the rest of it largely hurting. Keep in mind this is a theme park that is pretty much the exact opposite of the ideal theme park model (which would be the circular Disneyland idea), as Potter has turned the rest of the Upper lot into an awkward U shape. At least during the first summer you can't enter Potter to get to the other side of the park. The rest of the park is situated downstairs through a series of four escalators in a tiny parcel piece. Let's again add in the fact that some people aren't going to want to risk doing other things in fear of losing their RTP slot. They are called Harry Potter fans and they've literally come to the park to experience Wizarding World and do not care about ANYTHING ELSE. I've know lots of people who have gone to Orlando and never stepped a foot outside the Potter areas unless they were entering, exiting or waiting to get in.
The hilarious thing is that the entrance to Potter is this tiny little parcel. Okay, so you know how tiny and short-sighted the TF single-rider line is? Back when I was working the attraction the policy was due to the line crossing over the path to an ADA-mandated elevator, it could not go over 25 minutes in length. So we would close it and announce over the entrance PA to come back at a later time. They weren't allowed to line up outside the entrance, but aside from forming a line we couldn't really do anything about loitering. So they would just hang out in front of the attraction and all we could do is tell them not to. But they're guests, they aren't going to listen to that unless you've got security or crowd control restricting them. And we all know the area in front of TF is quite expansive, and it's rather unnecessary. Well, take that and minimize that by about 1/3rd the size at best.
So security/crowd control obviously is going to come into play, put a fancy rope there and designate this as a walking-only area. Pretty easy problem solved right? Not so much. Remember: Potter fans have only come to do Potter. Animal Actors? No. Potter. Despicable Me? No. Potter. Lower lot? --sorry does it have to do with Potter? No! This happens everyday at USF/IOA, but luckily Orlando has two parks and size to store these waiting people. At USH not so much, and now security/crowd control will have to manage every restaurant nearby (Springfield/Pinks/Mel's) to dispose of loitering people. Simple fix for Potterheads: grab some food, let it sit while they wait. So now you're going to have seating issues at these places, making it harder for the other guests wanting to enjoy the rest of the park, which by the way are about 75% of guests now unable to find somewhere to eat, making them eat in lines, walk in congestion due to the other Potter fans who aren't jerks walking around aimlessly and impatiently. USH just simply doesn't have enough space to soak all of this chaos up.
Not to say they aren't completely screwed. I can immediately think of a cheap fix: guests have two options when they receive their RTP...
1) Enjoy the rest of the park, come back during the alotted time.
2) Wait in the air-conditioned Globe Theater, which is playing the Harry Potter movies. Possibly some free water, and access to restrooms across the street. A digital clock sits on the wall, telling everyone the time so they can keep track. Briefly, the film is interrupted and Team Members will call over anybody for a specific time, and they will head as a specified group over to Potter with possibly a ten-minute early advantage. However you must enter the building at least thirty minutes prior to your return time, and of course must have a pass. The theater is massive enough to be filled with benches and is cool and dark. (they could also use Universal Plaza, but I'm not a monster)
That would take care of a problem that is sure to happen in the first summer, but I'm not Universal management so who knows? But the fact is that it will be a problem and there needs to be a solution or you're going to be in for a capacity nightmare in 2016.