On the advice of Cakewalk tech support I disabled "Enable Delay compensation" and as long as I have the LIVE button permanently enabled in the TI plugin, I seem to have got rid of the latency problem.
CW support also suggested configuring the TI plugin as a ".. tempo-based effect" in the Sonar Plugin manager rather than as a synth. It seems to be working OK as a synth for me: what does everyone else have it configured as ? And what practical difference does it make, in the way that Sonar handles it ?
Now all I need to do is fix the pitch drift problem when using Virus TI

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