I'm currently building the first G sitemap for my site. There are several thousand pages on my site, all dynamically generated. The sitemap software I'm using is currently up to about 4300 pages mapped. My indexed pages in G usually stands at around 700-800. I'm wondering a couple of things about once the sitemap is online. First, will G pick up all those other pages, and second, will it retain them once it does?
I don't know. it would be a good experiment. I presume you have google webmaster tools set up. if not then do this. I would then add the site map and see what happens. Post your finding it would be good to know
Do notice though that the number Google Webmaster tells you are indexed is really the number of URLs in the submitted XML sitemap verified to exist in the Google search index. (At least in the past you could reset the count by deleting/adding XML sitemaps, but your URLs would still be indexed in Google even if Google Webmaster Tools claimed it was 0) Anyhow, if I were you, I would be sure to submit an XML sitemap. It won't remove your existing pages from the search index, only help getting more indexed. (One exception could potentially be if you have lots of duplicate content, but that would be an extreme corner case.)