A (slightly unrealistic) comparison:
Max damage of Detonator if you cap fSTR, pDIF, with WS salade, gorget, reasonably difficult but not impossible to obtain gear, HQ staff vs. piercing weak mob:
Max damage of Slug Shot with the same gear set, feint on the piercing weak mob, angon, dia III, DRK roll, /WAR berserk (basically everything to cap your pDIF):
Max damage of Leaden Salute with max AGI build, max MAB build (+15), pluto staff, Anrin obi on darks day, target INT = 60, /RDM:
So for a merit level mob, especially colibri, the ceiling on slug is about 2x that of det and over 3x that of LS. But this isn't WS averages, this is if you debuff a mob, buff the snot out of yourself and want to pop a single epeen shot where your pDIF value lands right on the max (for slug at least, Det and LS won't vary as much).
If you increase mob level to 90, VIT to 85, inrease mob def by 50%, make RACC consessions for slug and INT to 80, but still have feint, dia III and angon:
Max damage of det (you can use about the same gear set as above thanks to the ACC bonus, swap out a little AGI for more STR since the fSTR cap is higher):
882 damage (45% reduction in damage over merit mobs)
Max damage of Slug swapping out some gear for RACC but still /WAR with berserk and DRK roll:
991 damage (68% reduction in damage over merit mobs)
Max of LS with the same build as above:
1046 damage (7% reduction in damage over merit mobs)
So, this is not 100% accurate, but it gets the point across. Slug has the highest ceiling, but as your target gets more difficult its damage output decreases very quickly and you need to rely on your party/alliance to keep its damage up.
Det is decent, consistant damage.
Leaden Salute requires a very good gear set to get decent numbers out of it, but it will be the most consistant damage (I'd argue the numbers above are about 20% higher than the average-above average LS build will get). In that case, it is very much like an uber QD. 300 QD damage on a merit mob is not super significant, but 300 damage on an HNM is better than some WSs. LS's 800ish damage on a merit mob is not spectacular, but 800ish damage on an HNM is respectable.
They all have their place.