I have done some extensive testing and here are my results
After 100 runs, I have inconclusive results as to whether guardian jewel outperforms facet
My test in case anyone attempts to replicate:
I took a sorceress with level 40 fireball + maxed synergies / level 40 fire mastery. I swapped gear gear between a shako with +5/-5 fire facet and a shako with a +10/-5 defender's fire, and ran 100 tests of killing norm a1 andy in p1
I timed each kill and averaged the results and got a statistically insignificant +/-% result in time to kill per run (1.372% faster with facet)
Therefore I cannot prove whether guardian jewel or facet is better, but the evidence suggests facet is better.
what about a +5/-10 defender though