What does Lightmap Pack's "Pack Quality" option do? What does the "Pack Quality" option do in the Lightmap Pack settings box? The option's tooltip says Pre Packing before the complex boxpack.
Packing quality controls how efficiently laid out the UV islands are on the texture. 

A low pack quality means there is more texture space wasted (but faster unwrap), a higher value means the UV islands are better optimized to use the entire texture, at the expense of unwrapping time.