I'm not running the game now so let's just say that your SoJ = 200 dmg. That means that Salvation(1/4 dmg) will heal you for 50.
If you were to ONLY spam SoJ(with Salv healing for 100%, and Reparation being off), you would do:
133.3 dps and 33.3 hps(due to 1.5 gcd rule).
Threat is now: 733.2 tps(from dps) + 91.6 tps(from hps) = 824.8 tps
2 piece justicar crystal will make Salvation heal for 75.
Spamming SoJ will:
133.3dps and 50hps
Threat: 733.2 tps + 137.5 tps = 870.7 tps
The difference is 45.9 tps, which means that it gave a 6%~ increase in threat gen.
Once again this is with only SoJ spam being taken into affect, no auto attacks, no attacks that proc a lesser Salv, and no procs off reparation.