We went during that week in 2015, and it wasn't all that bad, but we always aim to zig when others zag.
The worst thing we did was to make an ADR! BY FAR our WORST wait was the wait we experienced was our horrible mid-day wait to eat at Liberty Tree Tavern. They don't honor their end, and make us wait 2 hours past our ADR, but if we don't show we're charged a fee?!?!?! (Stick to Quick serve meals and you'll be fine, especially if you can simply avoid eating noon-1pm. The resort QS are especially quiet mid-day.)
Now we'd planned on seeing the MK fireworks - I want to say - on the 30th. But the most magical thing we experienced was a shuttle boat ride on - I think- the 29th of December. We just happened to hop on the WL shuttle boat (towards MK) as the MK fireworks were starting and happened to sit on the front of the boat. It wasn't the kind of thing you could plan, but just- wow!
Another kind of semi-secret treat to see that week is the personal decorations put up by guests staying at Ft Wilderness. It is campy, but fun. Unfortunately for us- we were staying at WL- the shuttle bus back to WL - never arrived. So we went with the flow and headed to the Ft. WL beach to see them. The beach would have been fine, but a small group of (intoxicated) oblivious folks near us kept blabbering LOUDLY through the whole show, and kind of ruined it for EVERYONE else. Folks that inconsiderate kind of baffle us.
In higher crowd times, it pays to use early/late park hours. Most of the year, 25min is our line limit. Holidays, we up our upper limit to 30min. If a line is over 30min, we keep walking.
We still managed to ride many attractions. Mid-day, we rode LM-Dumbo-Barnstormer triad with minimal wait-times and that surprised us.
The other really big work-around we did that REALLY worked well was to spend NYE night at a Universal deluxe hotel with Express Pass. US has all kinds of special events and high $$$ ticket meals on NYE, but we just picked regular park touring and a regular table service meal at Portofino. (We made the reservation just a week in advance.)
The NYE fireworks at US weren't as spectacular as MK, but we greatly enjoyed the much smaller crowds. With EP, park touring was easy. All our waits were 15min or less except Hogwart's Express, and we found our fireworks spot about 20min before they started. Even better, the walk from the park to our hotel was a mere 10minutes!