Distributed parallel rendering in Gaming or VR rendering VR rendering needs a lot of GPU power. VR SLI can help that. But, is it possible for us to use distributed parallel rendering technology to improve the performance dramatically? There is an open source software "equalizer" which could do that.

For example, put 4 or 8 GPUs in one PC server and disable vender's SLI driver. Use equalizer to do parallel rendering. Or, run equalizer on 16 PC servers with GPU equipped. 

So that we can get very high performance easily? Right?

Equalizer supports sort-first and sort-last. Since sort-last needs a lot of network bandwidth, we'd better use sort-first.
You are essentially correct in assuming that it is technically feasible to distribute the rendering computation workload. It is, essentially, a computation workload like any other; possibly even better-suited for parallelization due to its very nature: many similarly structured units running the same code path (i.e. vertices, pixels).

What you are forgetting, however, is latency. Having all the render power in the world is useless if the image does not arrive to the HMD fast enough not to cause discomfort to the user. Here is an article that explains the importance of motion-to-photon latency:

http://www.chioka.in/what-is-motion-to-photon-latency/