This isn't about performance of user space drivers but about safer/better drivers. Whether that driver offers an XDP interface or something else is irrelevant. Also, your kernel driver running XDP should also use the IOMMU for both safety and security (e.g., Thunderclap).
(The thesis features a performance evaluation to show that it doesn't get slower when used properly: hugepages are absolutely required; this is very different from non-IOMMU drivers where hugepages only boost performance by ~1-3% in the best case. Also, performance is simple to quantify so it makes for a great evaluation.)
Some context for this thesis/what I'm working on at the moment:
C as a programming language for drivers has failed from a security-perspective. Cutler et al. [1] analyzed 65 security bugs found in Linux in 2017 allowing for arbitrary code execution, 40 of them could have been prevented if the driver was written in a safer language. I've looked at these 40 bugs and found that 39 of them are in drivers.
I disagree with their conclusion that you should therefore consider to write the whole operating system in Go (that's just unrealistic in the near future). But we can at least start to write drivers in better languages as they seem to be the weakest point gaining 97% of the improvement for ~40% of the work. Getting a high-level language driver upstreamed in Linux is of course unrealistic, so user space drivers it is.
Network drivers are particularly interesting because user space drivers are already quite common there and there's a trend towards having network stacks in the user space anyways: QUIC is user space only, iOS runs a user space TCP stack. (Somewhat related: the future API between applications and the network stacks is TAPS instead of sockets, TAPS is also more friendly towards user space stacks as it's finally a modern abstraction for the stack.)
[1] https://www.usenix.org/system/files/osdi18-cutler.pdf
