I finally got stable MIDI timing with TIOS v3.3.5.02 (shown in the Virus as 3.3.4.00 when installed).
I know there are a lot of features which were implemented afterwards,
but since I only use external FX I am happy to have the typical Virus Oscs and Filters which is all I need from this Synth.
With TIOS 4.5.3.00 the timing issues start when the Amp Release
Envelope is higher than 0, also some MIDI notes get lost.
Have not tried any lower version from 4.x but just took the highest version of v3.
This solved the problems, no missing notes or timing problems anymore.
Hope this helps (not tested the USB Plugin setup, just plain MIDI with Audio Outs).