Just to clear things up.
- You don't need 10k affection. You just need to finish their entire quests / conversations. Some companions finish up at 9200 some at 9500. Of course there is nothing wrong with capping them at 10k.
- You get a buff for each type. Ranged tank, melee tank, ranged dps, melee dps, healer. They also give you a stat buff per type. These don't stack. You can't max out a ranged dps on 1 character and 1 on another. it only applies once.
- Also Human gives a presence buff.
- Also crafted "Leadership" armor as mentioned.