The folks over at Legitreviews have an excellent how-to for improving graphics performance on the Haswell i5 NUC (doesn't work for the i3). I walked through what they suggest and in theory it should boost performance. I decided not to try this on my i5 partly because it's doing just fine as is for all the games my daughter plays on it, and partly because I don't like to tempt fate. The method in the article would work but it would also increase the temperature of the CPU, especially when under load...you know, like when you're playing a graphics-intensive game. With no way to improve/increase cooling in the NUC it would make me nervous to run it hot. It would probably be fine to run your system like this, but just as with overclocking a CPU you should proceed with caution. I've never been a huge overclocker because I like my stuff to last as long as possible and in the world of computers heat is the enemy.
Still, if you want to squeeze every last drop of performance out of your i5 NUC, if you're an overclock-aholic, and if you like being a little "dangerous" you can find their article here.
One bit of (hopefully obvious) advice; read the WHOLE article. Don't just read the first part and then dive in...know what you're doing before you proceed.