Register pressure in Compute Shader
Lowering register pressure doesn't necessarily give you any performance boost though. I recently went through this exercise myself on GCN architectures (for a simple ray tracer) and reduced register pressure so that it increased occupancy from 2 to 4, which had no impact on performance. It's generally a good idea to reduce the pressure if you need to hide memory latencies, but it really depends on what the real bottlenecks in your shaders are.

Since you are working on a GPU ray tracer, you might get better improvement in performance by paying attention to the divergence of threads and try to improve this instead. For example by trying to group rays based on location and direction to reduce divergence in KD-tree traversal per wave. I'm not familiar with the paper you are referring to, and you may already have considered this though.