unsure if the lack of global physical to pierce is because of balancing concerns and/or “thematic” reasons
but i would like to suggest consider changing Sinister’s “increased % armor piercing” to straight up 80-120% global physical to pierce conversion instead
aside from then allowing more easily some other pierce builds to take form, it would also make it feel less sadge when you drop Sinister on a axe/mace or low AP gun 
- while then granting those items/this prefix that bit more awesome general utility