E.g. for the document linked from http://code.google.com/p/sumatrapdf/issues/detail?id=958 or page one from the document linked from http://code.google.com/p/sumatrapdf/issues/detail?id=962 the fonts are misrendered. Not calling pdf_makewidthtable for Type0 fonts seems to fix this issue without regressing any of the other documents relying on pdf_makewidthtable.
Agreed -- the font metrics of CJK fonts are usually very close or very different so the stretching doesn't buy us any legibility. The test to disable can be made on the existence of fontdesc->tottfcmap which is only loaded when substituting a CJK font with a truetype.