Since the Stardraw and DWG formats are both vector-based the results from exporting to DWG should be identical to the original data. I suspect that it's the View Resolution setting in AutoCAD that is at issue; for example, if View Resolution is set too low, circles might be rendered as a collection of straight lines. This does not mean that the object is not a circle, it's just that the rendering engine in AutoCAD is set to break curves into lines for display purposes only.
Try using the VIEWRES command in ACAD to change the display resolution to a higher setting.
If this does not solve the issue for you please email the original .S07 and exported .DWG files to
techsupport@stardraw.com and we will be happy to investigate further.