benchmarks say its 50% faster, however it has 2x as many cuda cores which is important for apps like adobe. do you need it? depends how much premiere is using what you have now, odds are you are ok. you can check yourself with gpu-z. GPU-Z Video card GPU Information Utility
So what John and RoninEdits are both saying ... it depends. IF your current GPU is the slowest bit of your machine, therefore slowing down the other parts of these complicated systems, maybe it's worth the money to upgrade. IF your current card is as or more capable than the least capable other part of your system, probably ... not going to see enough benefit for the cost. If something else is way below the other parts of your system, you won't see the benefit as you couldn't be utilizing your current card's capabilities.
So ... watch GPUZ while working. Are you regularly maxing out your current card for good bits of time? If so, upgrade that. If not ... perhaps download the bench-testing setup from the Tweaker's page and see where your systems problems are.