I agree with Frank. I just go ahead and record the audio from my external synth into SONAR. At one point I tried using External Insert and bounce, but it was really just an experiment and didn't save me any time, since I still had to turn off "fast bounce" anyway. It takes as long as a real-time recording.
What's nice about having an interface with multiple in/outs (MOTU Ultralite) is that I can record all of my Motif parts in one swell foop, so that does save multiple passes and consequently time.
P.S. @Frank: I noticed in another thread you mentioned using the SPDIF clock if you're using a SPDIF connection. I hadn't thought about that before, but I use both SPDIF and analog connections when I'm recording into SONAR, and I haven't noticed any timing problems. I can try switching to the SPDIF clock next chance, but I haven't had any problems so far. Does it just fall back to the lowest common denominator, if you know, or should I do some "serious testing"?
Thanks-
-Susan