I am currently using a Radeon Sapphire R9-290x 4GB purchased in 2014 for editing in Premier Pro CC. Would I notice a significant speed improvement upgrading to a card such as the GTX 1070?
it would depend if your system is bottlenecking on the r9 290x or not. if you aren't using enough gpu accelerated fx and features, premiere won't be pushing the gpu very hard. you can check task manager for cpu usage and GPU-Z to check gpu usage.
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