We carried out some simple tests of Nginx on the rump kernel a few months ago. Performance is approximately similar to running on an "ordinary" operating system +/- 10% depending on a variety of factors.
It's worth bearing in mind that the Rump Xen backend isn't well optimised at the moment, so there's quite a bit of low hanging fruit that could probably boost performance quite a lot. I think things like multi-page netfront, more careful rx/tx batching, and probably scheduler measurements will have an impact. The focus right now is on getting the individual applications to run reliably and robustly as unikernels when recompiled under Rump, and upstreaming patches where applicable.