We experimented with HHVM, had a preview version of it very early on but ultimately the extension support was too bad and the performance gain just not impressive enough to justify moving away from the existing stack.

We didn't have a research-oriented team per say, but myself and a colleague (I was Lead Dev and he Senior) did spend a lot of time experimenting with things like Redis (back when it was very early and "unproven"), varnish, PHP engine tweaks, custom extensions, ... We still did feature work and regular stuff too.