Be aware that you will never replicate the functionality of nightblade cloak with potions.

In addition to the obvious (15 sec duration with 45 sec CD so you can't stay invisible permanently like a NB can), keep in mind potions do not suppress DOTs on you - any DOT will remove the invisibility next time it ticks - and they also do not cause projectiles already in flight to miss you, so that frag on it's way when you quaffed the potion will kick you right out of invisibility as well.

The potion is generally useful in two scenarios - when you are solo and a zerg spots you from a long distance, an invis/speed potion enables you to evade them, and it makes it easier to get inside your own keep that is being sieged when the attackers are intercepting reinforcements.

But being already in combat - forget it, unless you only want to use the potion for the speed boost.