While performance is generally very good with XML files, also with big files, i would mostly worry about frequent writes to huge files - that has a high chance of data corruption and just simply wears of your disks.
Especially things like Content Versioning of the Newsletter statistics has a lot of writes on files that can grow quite large - i had one site with xml-files of over 200 mb.
Its also easier to start out directly on sql. While it sounds good on paper to switch from xml to sql in 10 minutes, its in my experience never the case on any decent size solution. It's easy to make Linq queries on xml that can't be translated to sql and you end up spending time tracking down errors and fixing them. Easier just to start out with sql up front.
Especially things like Content Versioning of the Newsletter statistics has a lot of writes on files that can grow quite large - i had one site with xml-files of over 200 mb.
Its also easier to start out directly on sql. While it sounds good on paper to switch from xml to sql in 10 minutes, its in my experience never the case on any decent size solution. It's easy to make Linq queries on xml that can't be translated to sql and you end up spending time tracking down errors and fixing them. Easier just to start out with sql up front.