the problem with VST mixers (and i write ALOT OF Pluggins), is that the audio must be directed from the internal channel thru your set of eq's and busses and effects and back out to an audio out on your host program.

VST's in general are CPU hogs, and a home grown vst mixer will be devastating because it is essentialyl a gigantic box of dozens of smaller vsts.

i do get a kick out of making my own synths & samplers though :-)