I was talking more about SotR uptime, rather than the usual Seraphim/DProt/Sera/DProt/... Even with Sera/DProt active, it's not unusual to be hit very hard during the Sera build-up. I get your point though.
I have no idea where you saw that 20k TMI decrease from switching to EmpS though. Numbers on Theck's blog according to his simulations are as follow:
Eliminating the rest and just taking into account the SoR simulations, that gives us :
EmpS_SW = 138k TMI
Sera_SW = 142k TMI
Maybe you got that from a EmpS_SW vs. Sera_HA comparison? That I could understand since Sera_SW is quite unusual.