So Companions can wear the finest gear available (other than relics aka trinkets). It's pretty easy to get great basic gear via PvP warzone commendations for your companions. From there it's about mods (gear upgrades). I tend to equip my companion first, since it makes a huge difference if the companion is worth his/her salt or not. With subpar gear, companions just kinda do nothing much, other than cost time regging them back into fighting condition.
With equal or better gear though, they can perform extremely well. I found my range DPS companion to outdo most DPS players of equal level. At first my melee DPS companion was worthless (dualwielding marauder archetype), now with great gear and lots of accuracy... quite beastly! Definitely the most important lesson I learned. To put as much or more effort into my companion gear. Makes life so much more enjoyable.
That said, I've another 1000 commendations on the bank. I ponder if I shall wait to get my ranged tank companion or if I shall give Khem Val the full make-over with PvP gear. Somebody know a place to buy a slotable vibrosword?