There's likely to be new GPUs as well (as discussed in other threads about the next iMac) as there are usually GPU changes with news systems.
The question is will it be one we already know about - if so don't expect much more than 5% or so performance improvement. Or will it be one that is new for the iMac like the 680MX was - in which case it might be quite a nice boost.
The other probable improvement that you would be interested in is that the Fusion drive and the Apple SSD options are likely to use PCIe based SSD for better performance.
If I needed a new system right now, I would probably not hesitate in buying the current system, however if you can wait, just but the next iMac when it's released whatever performance improvements there are.