I just saw a little bug with this plugins working with.
If someone visit (before) sitename.multisitename.com/sitemap.xml your all address generating for this url and then serving from cache file.
This is a bit risky.Why?
Because if you are using sitemap and domain mapping plugin and first you create your sitemap with subdomain url that adress is wrong. (When you visit address, you can redirecting mapped address but I guess search engines don't like it)
Edit:Tried with latest version of these plugins.