This would be extremely helpful. If there's anything I can do to help by testing, etc please let me know.
Like I mentioned, even though Multi Threaded sending wasn't "supported" it still worked in version 6.15 by appending the startqueue parameter like was done in AcyMailing v5. Something changed though in version 6.18.3 and it no longer works and therefore sending speed has dramatically slowed.
Again, please let me know if I can help in any way to expedite this feature.