With the DLC, Aurelia (the Baroness) has "The Huntress" skill tree that has a lot of sniper-related goodies.
Tier 1: Only The Best gives you +20% bullet speed and +1 magazine size for sniper rifles (per level).
Tier 2: There's a one-point skill called Long-Range Killer that gives bonus damage based on your distance to the target (farther away = more damage). Then there's also Warning Shot (when you miss with a Sniper, you get a few seconds of bonus to Fire Rate, Accuracy, Critical Hit Damage, and Weapon Swap Speed).
Tier 3: A one-pointer called Silver Lining (kill with a critical hit, add a bullet to your magazine).
Tier 4: Magic Bullet is a one-pointer that regens health for you when you crit with a Sniper Rifle (closer = more), plus it also creates a healing nova at the location of the sap you killed, healing your allies (including second winds!), and that one scales opposite (further away = more). Then you get a choice of either I Never Miss (Each hit with a Sniper Rifle adds a stack, two if you crit, each stack adds crit damage with sniper rifles, but you lose the stack if you miss) or Wait For It (adds a stack each time you miss, when you hit the stacks convert to extra damage).
Tier 5: Nothing specifically sniper-related here.
Tier 6: Custom Loads is the end of the tree, and it adds the ability to add any elemental type to your sniper rifle when zoomed in (there's a scrolling wheel mini-game type thing involved).
If you're playing Co-op, the Contractual Aristocracy tree has some nice bennies, but nothing specific to sniper rifles, so it's probably best to pick these up after you've maxed out the other tree. (Duchess is probably the best "splash" - it's Tier 1 and gives stacks of Accuracy bonuses)