It's not 50% OC over reference because that's misleading.
Out of the box, 980Ti reference reach 1.2ghz actual boost clocks from various sites I've seen, including AT's review.
If they OC to 1.45ghz, that's 250mhz on top. ~20% OC. If they reach 1.5ghz, that's a 25% OC.
Custom 980Ti models reach >1.3ghz boost out of the box, that's why they are so much faster than Titan X.
http://www.guru3d.com/articles-pages/gigabyte-geforce-gtx-980-ti-g1-gaming-soc-review,1.html
An example here, its set for nearly 1.3ghz default boost, which usually translate to higher actual in-game boost.
They can be OC further, there's usually 10-15% performance left, depending on the models.
http://www.techpowerup.com/reviews/EVGA/GTX_980_Ti_SC_Plus/33.html
http://www.techpowerup.com/reviews/Gigabyte/GTX_980_Ti_G1_Gaming/33.html
It's why Fury X looks crap compared to these custom 980Ti offer great bang for buck comparatively and has excellent OC potential eventhough they already are highly clocked out of the box!
For Fury X to reach 20% OC, it needs to hit ~1.25ghz which is just not possible without vcore tools. 25% OC on Fury X is 1,312Mhz, which is unlikely to happen even with vcore. GCN peaks around 1.25ghz when we examine 7970, R290X.
Without major driver improvements, OC v OC, Fury X still loses with vcore tools.