I thought I'd try to see if i could get this to work...and yes it does work, but it takes a long time for the video to start playing. It seems to be dependent on how far in you change the start time. I actually gave up, went to check the mail, came back and the video was playing.
Have you tried working with QuickTime Pro? Trim to where you want to start, save an alternate version then import it into Sonar?
Something I have been doing for years is creating an AVI file from Vegas that always has the 1st frame of Picture or music start :10 secs in. I get so many revisions with so many different start times and so many different codecs that I just standardized my work flow to save me thinking about any of this.