XFS or ext4 filesystem mongodb server on Ubuntu machine?
XFS is better in general with WT, as the MongoDB production notes suggest.
I ran performance benchmarks comparing XFS with EXT4 for MongoDB on AWS EC2 to find out exactly what you were wondering about. Here are my results. The observation was that XFS is useful when your machine has multiple cores and fast disk that XFS can utilize. On low to mid end systems the filesystem did not make a difference.
As you have said in your question, you should use XFS for MongoDB but for the general system is better ext4, so what I recommend is to have two partitions (or better two disks) one with ext4 for the system and general use, and one with XFS for MongoDB and storage of big files.