Could be bad -- Sunday afternoon/evening during the summer means lots of vacation pax and slow security lines. You may be late enough that you can avoid the worst of it, but IME, that's one of the riskier times. Checked bags aren't that much of a factor because the real problem is the security mess.

In general, I'd probably shoot for 75-90 minutes before the flight if you want to make sure you'll make the Sunday PM flight. At 1 hour before, I think you'd still have pretty good odds of making the flight, but if it happens to be one of those really bad days, you might not make it.

The elite line discussed in this thread should be in effect by then (knock on wood), which may help a bit. No way to tell how much benefit it will be since we haven't seen it yet.