Does a maxed dark sight stack with haste?

When your DS maxed level , your haste won’t lose movement speed after dark sight

 

Darksight wont cancel anything since it has no effect on your speed whatsoever

 

To answer your question - no, you don't slow down. That's what maxed dark sight provide; you get "regular moving speed" which is whatever speed you currently have. E.g.: With maxed haste you get 140% speed and 120% speed, so activating maxed dark sight will allow you to keep this current movement speed/jump. Note: Maxed speed is capped at 140%, so you can't combine speed potions and maxed haste. Maxed jump is capped at 123%, so you can combine maxed haste and +3 jump from equips!

You should always max dark sight by 1st job (to prepare you for LPQ's thief portal; you won't slow down with maxed DS). A lot of people make a mistake of maxing nimble body and keeping dark side at level 10. This should keep your nimble body at level 10 for now. Finish leveling nimble body in 2nd job after maxing haste, critical throw, and claw booster. Then put your spare SP into drain until you hit level 70.
PS: A good tip for new sin/bandit players is to max haste first as soon as you hit 2nd job at level 30. You will make LPQ so much easier and faster, and thus level up quicker! Plus, your party will love you. You only need damage from critical throw if you plan on grinding, but it is much slower than LPQ at lower levels, so max critical throw only after maxing haste! You can also add 9 SP into claw booster after maxing haste, for a ~1:30 min duration. It's up to your preference, but do realize that the booster skill speeds up your attack speed at the same rate for all levels of booster (this applies for all jobs). Adding SP into booster just lengthens the duration of booster and changes the HP/MP cost.

 

Fwiw, I don't think it's a mistake to not max dark sight in 1st job since thief usually don't finish that stage last, and sometimes when you're really unfunded you actually need the accuracy from nimble body to hit alishar.

 

tl;dr version: no.

 

You'll max drain with 3rd job points if you want max dark sight faster. Shadow web and shadow meso (throw mesos instead of stars, not sure if this is the right name) are 100% useless so put those points into other useful skills like drain.

Nimble body is a nice to have, mostly for lower levels.