well you could use a hybrid approach.
1. save original epub or set that option in calibre for epub to epub convert.
2. run the calibre convert
3. open the result & make a note of what styles were deleted
4. delete that epub file, restore original , manually delete styles noted in step 3.
cold be faster than lots of find / count operations ?
I'd love to see a sigil enhancement or plug in though - that would persuade me to move up from version 4