Advanced Cloaking gives more mana sustain, and more mobility while stealthed.
Covert Mission is a really good bribe, split push and annoyance!
Perfect Shot gives you resets on your main damage ability.
Precision Strike is an amazing tool either for combos, finishing off a target or a structure, clearing far away waves to get experience and lane pressure, or defending from those.
Psionic Efficiency makes you more consistent and you can poke from really far. If you miss your Snipe, you only need to hit 3 times to get back at maximum stacks now.
Explosive Round will increase your waveclear and AOE damage in teamfights, from a very long and safe range.
Precision Barrage gives you even more global potential with a second charge of your ultimate and a lower cooldown.

At level 1, you can switch to Longshot which gives you more range for your second ability, making you safer if the enemy cannot dive. You will miss the extra mana regen though.

Your Snipe stack will not reset if you use it on minions, mercenaries or structures, it only resets if you don't touch anything.

Covert Mission makes camp respawn sooner if fully taken with bribe, this will help double efficiency as camps give experience, taking them more often gives an experience lead possibility.

Minions killed with Precision Strike will grant bribe stacks for Covert Mission.