I've opened a very large assembly from Solidworks into Composer and it has quite a low resolution mesh. This is both visible in the main window, and after exporting as a technical illustration.

The options in  Composer which I have found was:

• Input - Relative - Chordal error - high quality
• Output - Geometry compression - Low (high quality)
• Technical Illustration - Lines - Refinement - high

Using the Image Quality slider for the document in Solidworks doesn't seem to have any effect on importing (even though, weirdly enough, having perspective on/off the camera in Solidworks does have an effect in Composer).