your right
webtools/spell.php?id=9117
is taking preference over BoF foci
With my chromaic arc items off, it does increase the dmg. Which makes sense, since BoF is a "focus" effect. However, the Faerune and Brilliance of elder foci are "worn" effects and should stack, however they don't work.
tested webtools/items.php?id=17720 which has brilliance of elder, and webtools/items.php?id=10603 which has Faerune 3 and those do not increase the dmg, with or without the chromatic arc items equipped.