Is there a performance impact from using a user space file system vs a kernel space filesystem? I ask because I have a friend that refuses to use user space file systems because of a performance impact. I've done some research but cannot find any benchmarks of user-space vs kernel-space file systems.
There is a research paper out there titled, Performance and Extension of User Space File Systems by Aditya Rajgarhia (Stanford) and Ashih Gehani (SRI) that goes into exactly what you're looking for. It is easily locatable.
The conclusions reached match my own perceptions of this that user-space file-systems (FUSE specifically) do perform measurably worse than kernel-space filesystems. They go into much more detail about their findings.