i haven't pushed the system to a creasy amount of Vstis+EQs etc etc yet, but everything i do ( mostly between 30 tracks audio @24bit/44.1) plus some EQs and a couple of FM7s is really no big deal for the 4670 ( maybe 20% or below on the SX performance windows in average..). i m guilty also not to have spent a lot of time doing music but :
i do a lot of midi with my (real) synths ( old habits? haha) and the way it is set up is that i kept 4 synths connected to my Adat (only used as a AD converter+digital sync > RME > 4670..so i still have all my RME inputs and outputs free for now ( and will use some of them for my external fx boxes and also with a mic pre in the future..)
the 4670 as DAW system is realy great..any future 46xx should be even better hopefully if you do a creasy amount of VSTis..