They're called the "laws of physics," not "the suggestions of physics," for a reason
I've worked with many different stretch algorithms from many programs, and there is a limit past which you simply will not get acceptable results. Different algorithms react differently. The offline stretch algorithm in SONAR is actually very good, after you render it. zPlane makes excellent algorithms as well but the differences are minimal, if even noticeable, in terms of quality.
It's always better to shorten rather than lengthen. When shortening, you remove data but to lengthen, you have to create data that never existed. That's a tough task, even for today's computers and DSP. You may get better results from splitting the clip, lengthening both parts, copying the tail, then doing multiple crossfades of the tails. it's hard to recommend specifics without hearing the audio, but the latter is how I
sometimes manage to do "impossible" stretches.