Well, The DSP mixer allows for much more grouping and outputting before the mixer stage. If you're stuck with 4 outputs, you can atleast get 16 stereo channels of subgrouping BEFORE the output stage to your mixer
yup there is a bunch of grouping in the DSP mixer for sure, but surely with regards to VSL, you can atleast group specific instruments together in the DSP Mixer before the output stage. With specific panning associated to those groups of instruments (take strings for example, short bows and long bows, and effects panned to the specific places you need via a grouping to a stereo DSP mixer channel)
There's no way to get true stereo panning control on every 64 channels of giga (which you only need if you're loading a completely different instrument, with completely different panning placement on every 64 of the channels. I find this pretty hard to see happening, especially with a library like VSL.
You can do the scenerio you're talking about. The DSP mixer is a set of 16 stereo mixing channels that can have any MIDI channel routed to them. So you can have the cymbals routed to DSP mixer 1,2 - the Kick and Snare routed to DSP Mixer 3,4 - and the third instrument to DSP Mixer 5,6. Then set each of these DSP Mixers to output to output 1,2.
The DSP Mixer is like having a hardware mixer in line BEFORE your outputs. Which makes the outboard mixer redundant, not the DSP Mixer.