The only exception that has worked for me is for common variations--i.e. when searching for spinal disc information, about 1/3 of users search using disk instead of disk.
(This might also apply to regional usages for spellings as in UK/Canadian/Australian English words (program/programme, color/colour, etc.)
As far as doorway pages--here in the US the major search engines' ability to scan for duplicate content on a sub-page level (by paragraph, and possibly even by sentence or sentence groups) has effectively eliminated the benefits of using doorway pages. On real competitive keywords, I have done work for clients optimizing doorways for specific search engines (and using meta robots and roobts.txt to block undesired search engine spiders/bots from those pages...) but it's a real headache to create, and a significant nightmare for the poor soul who has to regularly update the site content! (IMHO the client loses more in costs than gained by the slight traffic increase due to engine-optimized pages...food for thought!)
HTH!
-Tertius
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