To be honest, this build is a lot weaker than the burst nova by far. However, it does deal more damage over time if you can weave in an out through your enemies and just keep dealing damage by throwing out constant snipes and AAs.

Lvl 1: Psi-op because it gives you range and reduces the CD by 2(therefore actually becoming a snipe).
Lvl 4: You're going to be getting Perfect Shot so Psi-op can be useful and refunds mana as you constantly throw out snipes.
Lvl 7: One in the chamber because you're going to be throwing out so many skills.
Lvl 10: You can go for either. Choose wisely. Personally PS can deal a lot of dmg during team fights and makes you more useful.
Lvl 13: Lethal so you can scout and deal extra dmg with skills.
Lvl 16: Double that decoy. Double that dmg.
Lvl 20: Okay, tricky part. If the enemy team does poses no threat and can't get to you, go ahead and get the upgrade for PS. However, the blink allows you to escape. It's a plus to get blink because of how squishy Nova is to begin with.

This build isn't for everyone. It's honestly a fun build that only works with certain team comps. Nova is known as a burst assassin, but to me, she can be so much more. Honestly, if the enemy team has hecka tanks, you won't be bursting them down. Another thing is that burst Nova is more early-mid game while this build is completely late game oriented.