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Though a sitemap is a good thing to have, do you know why Google doesn't completely crawl your site through the standard navigation?
How old is your site? It genearlly takes Google a couple of months to get into gear and do a complete crawl of your site, whether or not you have a sitemap.
What is the PR of your site? Sometimes google just doesn't have time to crawl all the pages of sites that are way down the list. Even a site map may not help all that much in a case like this. If your home page is <PR3 you better get some more links. Heck, you should get some more links even if you have a PR7.
Do you have any external links going to your deeper pages? These can really help with crawling and ranking as long as your navigation works well.
If you take care of these basics, you should not have a need to have a site map that lists every product just to get crawled.