Are there any SEO benefits to serving up a compressed sitemap instead of just plain XML? 
Are there any SEO benefits to gzipping sitemap.xml?
seo;sitemap;xml sitemap;gzip
There are no SEO benefits to serving a compressed XML sitemap over serving one that is not compressed.The advantage of compression is simply to save bandwidth and the time it takes to download. (If your sitemap is huge.)Note that the limits for the size of the sitemap are the uncompressed size (ie. 50MB uncompressed for Google).