There's a _lot_ of variation based on the buffs you get and the debuffs on the boss, and raid boss fights are significantly different than heroic bosses. The default fight in simc is Patchwerk, which is a pure DPS race, as well.

One of the better uses for this stuff is gear choices. If I swap out this trinket with that one, is it net better or worse? It's not perfect, but it helps a bit.

Also, it seems unlikely that your all-guild 5 man boss fight lasted 300 seconds and allowed you to use all 25 man raid buffs.

Affliction doesnt do burst damage, in a 5-man the mobs all die before you get up to full DPS. Try fighting the target dummy and keep one eye on recount to see how the number changes through the fight. Personally I run 5-mans in destruction spec, afflic is better for long fights where you get maximum benefit from those big drain soul ticks below 25%.

Hell, I have a hard time getting destro up to full speed in 5-mans, beyond spamming Rain of Fire. Maybe on bosses, but heroic trash tends to be so trivial these days, until it isn't, like the Frozen Halls..