Yes, so you have a CPU upgrade (which main focus is laptops and ultraportables) that will possibly give you 10% more performance, which is not really noticable - unless you run very demanding work on your iMac all the time.
Also the 780M is just a rebranded GTX680/680MX graphics chip, if Apple decides to use this card. Yes, you'll get 4GB of VRAM, but the GPU performance will be a much bigger issue anyway in 1400p with the upcoming games. If you're waiting for a 780MX, remember that the new GTX780 is a more power hungry card, and thus produce more heat. This is a problem with the slim, closed design of the iMac. As such an 780MX needs to be underclocked quite a bit more than the 680MX (vs GTX680).
Because of limited performance upgrades for the iMac this year, Apple might wait with releasing the next iMac model until Thunderbolt 2 is available. As such you could end up waiting for several months. As always, buy now if you need it now...
The only real improvement IMO is the new PCIe SSD, but again has anyone done a comparison in terms of IOPS and real life performance improvement?