My recent 2070 Super is hugely more powerful than a K5000 (I had to check as I didn't know at all your card). But I checked with my old GTX 1060 and even that was hugely more powerful than a K5000.
So it's hard to guess. But you still have 1536 CUDA cores which should help a bit in GPU rendering mode. And no RT cores of course.
The best answer I could give : depending on the result expected, you should calculate with both CPU then GPU with a low maximum sample (about 50 ?) and compare at a pixel level both your renders in Photoshop. Then if both look similar and the GPU one was immensely faster. Then... go for GPU with a higher sample value for your final render
