It will be a large upgrade. Expect 60fps for most games at 1080p and high settings. It probably won't stay at 60fps with Ultra on very demanding games such as Witcher 3, but for all intents and purposes, you'll have more than enough power for 1080p gaming.
See here for some benchmarks: http://www.gamersnexus.net/hwreviews/1984-amd-r9-390-380-benchmark-review/Page-2