My test is not a latency test,,, it to test the accuracy of the offset that Sonar calculates so your tracks will be in snyc when you overdub.
Because of latency Sonar needs to do this.
Sonar needs to calculate latency of the system and driver and adjust the playback so that overdubs are put down in sync with the existing tracks. If your input latency is say 10 ms your overdubs would end up being 10 sec late on the time line. Your output also has latency so what your playing along to has to be factored into the equation.
So the audio driver seems to be what Sonar has to rely on to figure out how much it will move things around. If the driver lies to Sonar it will result in the tracks being out of sync. This is not Sonars fault, it can only do what the driver is telling it what to do.
So the loop back test is more or less a way to test the accuracy of driver.
From what you say your system is working correctly.
The test is simple as explained above.
Use a MIDI drum pattern.
Trigger any soft synth drum. Bounce to audio. At this point you can see that these line up correctly.
Then send that audio out of your interface and use a short cable to bring it back to any input.
Record that input to an audio track and zoom in to see if the new recording lines up with the audio from the original. simple. Sonar allows you to adjust the offset if it is wrong,, but myself if it's wrong you need to buy a better interface.
in my screen shot
Top track MIDI patten
2nd TRack Audio bounce of midi
3rd track ASIO normal buffer setting loop back recording
4th track ASIO highest buffer setting loop back recording
5th track WDM loop back recording