Like others have said, 10/5 Reaper is +80% gun damage against enemies with >50% Health. Keep in mind that some enemies (e.g., Hyperion Snipers) have way more Shields than Health. Reaper will give you its full bonus the entire time you’re chewing through the Shield, and then the first half of their Health bar as well! So in practice, any shielded enemy will give you the full boost for more than half of the total damage necessary to kill them! This benefits SMGs too, but really shines if you ever switch to a higher damage-per-shot gun like a sniper or shotgun.
Since you plan on using mainly SMGs, I’d recommend the Legendary Cat which will buff Reaper among a lot of other really good DPS skills, and gives a HUGE damage boost to SMGs.
As for relic, I recommend using a Corrosive or Incendiary Bone of the Ancients in order to boost you damage (multiplicative after all other boosts are added… even Reaper!). If an area has mixed Flesh and Armored enemies, I’d probably go with the Corrosive Bone of the Ancients since it will boost the amazing powers of Cloud Kill as well as any Corrosive SMG you equip.
Finally, Blight Phoenix does absolutely crap damage. (e.g., 1/5 points at level 64 was only doing ~1800 Incendiary and ~4800 Corrosive damage per second to an Armored enemy with a Corrosive Bone of the Ancients), but it does make enemies stumble when they first take damage from it. That’s why I’d only recommend putting a single point in it, even though it makes you look totally badass!