Well from personal experience (no number crunching). We have one enhancement shaman. He is usually in group with: 1-2 Rogues, 1 Warrior, 1 Hunter and or 1 Feral Druid. His group gets blessing of salvation no questions asked(whenever warrior is not in the group)

. Because his only way out of agro gain is Warrior's Feign Death.
Whenever we have him in the raid, his group does exceptional DPS, I've heard rogues say on vent that if he wasn't in their group, he(enh. shammy) would be be on par with their dps but this way he boosts rogue's dps and beneficial part is that totems move with him. Its also nice to have enhancement shamman on Shade of Aran.