Str 14, Dex 18 Ranger 6
BAB +6, Dex +4
+1 composite +2 longbow
Power Attack for 6: +5 attack/+0 attack, 1d8+9 damage (average 13.5 on each shot)
Rapid Shot: +8 attack/+8 attack/+3 attack, 1d8+3 damage (average 7.5 on each shot)
Manyshot: +6 attack for 2 arrows, 1d8+3 damage, precision damage applies only once
Power Attack for 4, for an even comparison: +6 attack / +1 attack, 1d8+8 damage (average 12.5 on each shot)
If the Manyshot hits, you've got 15 damage on average, not accounting for crit skew. If only the first Power Attack hits, it compares at 12.5 damage.
I'm really lazy, so a quick search brings me only Pathfinder monster guideline thingies that say average AC at CR 6 should be 19. Anyone got a more relevant source?
Hmm... I'll go through the SRD, calculate average and standard deviation and other relevant things.