To bring us back to the OP question, here's what I do.
Even though MOTU offers a limited, but VERY low latency monitor console, I still prefer to track, say, vocals through the Echo in Sonar.
The Echo latency is high when the buffers are set high, but also when plug-ins require compensation for their own processing delays. However, I wish to use compression when tracking sometimes. It allows a singer to force their vocals a little less sometimes, or just dig into the track a bit more, where timidity is a problem.
After lowering your buffers to 256 or less, click the PDC button to disable the plug-ins. See if this improves the Echo latency. If it does, uncheck that button and manually disable all the plug-ins you really don't need for tracking. Only enable the plug-ins you want to use to track.
Also, I've noticed at times, Sonar will intermittently exhibit far more echo delay feedback than other times. If I hear echo delay feedback, where I haven't heard it before, I can try a few things: Merely stopping the Audio engine (waveform button under the counter in the Control Bar), re-enable it. Or, I may disable the Echo button and re-nable that. Other times, I can stop the playback/record function and start over. I don't know what gets Sonar in this state, but it's still a problem I deal with regularly.
you may also try freezing all your other tracks, then click the PDC button.
A feature I've requested was to provide the Sonar user the ability to freeze the entire project with one click, so that you have a temporarily processed mix, and allow you to save your valuable CPU power and reduce latency by allowing your "armed" track(s) to use active plug-ins. This way, the rest of the mix plays like a single stereo track with no plug-ins. The only active plug-ins are on the track(s) you have armed to record. I don't know if that's even being considered, but I felt it was a good idea.