I do understand why I have dropouts, I'm asking for too much. The thing that bothers me is that I can receive way more outputs from my slave VEP (over LAN) to Cubase, with no problems whatsoever, than I can re-route with audio input plugins. That's many outputs going through LAN, so for some reason there is no bottleneck here, even though this is the actual network communication of so many audio streams. The only problem is the re-routing of Cubase to master VEP, which is localhost. My guesses are, that the re-routing is kind of 'outside' of Cubase, and every audio input plugin between the two VEPs is continuosly sending data, just to keep its link open, or the optimisations of Cubase, as mentioned by Dietz, are interfering.
I am already using Expression Maps for as many instruments as possible. To roughly some up my 300 outputs, these are 300 Mono outputs, VEP always grouping them in pairs, so it's 150 instruments basically. On that slave VEP I've got all my Berlin Woodwinds (14 instruments), all of my percussions (easily 30 instruments), Piano, Harpsichord, Harp, Organ, Choir and the lot, Guitars, Cinematic Soundscape stuff, Heavyocity patches etc., which sums up to 100-150 instruments. My master VEP basically has Hollywood Strings + Brass, Dimension Strings + Brass, which is also 60 instruments (with place holders for Dimension Strings Violas and Basses). I cannot swap these to the slave VEP, because especially PLAY loading times needs my SSD.
The only solution I can see is grouping my slave VEP instruments in a slightly unflexible way, e.g. grouping all Berlin Woodwinds into one woodwinds output section, grouping the entire Percusion into one output section, etc.. Then I'd have my 20 outputs at max which should work for 'audio input plugin' effects. Disadvantage is that every Oboe, Clarinet, Flute etc. is placed at the same MIR position. But if that's the only way around, then I guess I'll have to do a compromise. (which is what I'm going to try out and post my results) Maybe if I get myself a new soundcard at some stage (with proper drivers), this issue might be resolved, but I wasn't planing on doing that for some time.