Hey there. Ty for the reply! The conversion from the weapon will convert the flat damage.
This char must be even stronger now on patch 1.3 because menhir’s bulwark now converts 100% phys to fire.
About conversion stuff:
This topic can help a lot understanding the conversion mechanics of the game.
About DPS checking, there was a mod useful for that until the last patch, the DPyes. But i don’t know if they already updated this mod to 1.3.