Interesting results. Particularly they barely found a speed-up of newer compared to older LuaJIT versions (rather the contrary). Maybe they should have used the Are-we-fast-yet suite instead of the (random looking) set of microbenchmarks. I did measurements of Lua and LuaJIT some time ago based on Are-we-fast-yet and saw significant differences in LuaJIT performance (see http://software.rochus-keller.ch/are-we-fast-yet_LuaJIT_2017...).
I also compared different PUC Lua versions in an earlier measurement (see http://software.rochus-keller.ch/are-we-fast-yet_lua_results...) and found similar significant differences between versions.
As someone who has shipped Lua as a solution to many an embedded dilemma, this is highly interesting work.
I wonder if there will be motivation in the future to address energy consumption in future JIT work .. in fact I wonder whether other languages are going to face a similar optimization path. It would be grand to see progress being made on this at a more general scale. I'm looking at you, Python ..