So, few tips on reactor. Discordance my lower the damage of ORB... but the Reactor Set Affix is independant on the Orb Damage itself. Instead, it functions based on your MH (MainHand) damage. So, the higher the MH Damage (NOT DPS) the better each Arc Tick. Now, with that being said, Affixes other than reactor will stack HUGE proportions of damage onto Arc. Plagued gives poison DOT (Damage Over Time) a boost if Im not mistaken, and Poison Damage +xxxx or Poison Damage xx% affixes would help, along with the poison Elemental Critical affix known as Toxic.
The reason this set is so functional, especially at lower levels (< 300), is because discordance allows for the swapping of cooldowns. The wand could have Amber used on it to gain the Orb Skill, and then you would shoot Orb as fast as you did and for the same skill cost as Barrage assuming you have a Discordance mythic affix on a peice of gear. The damage and dps of this build comes from the fact that Arc has it's own damage calculation, and also applies elemental DOT to multiple enemies rapidly. This means Orb is merely the vessel that your DPS is carried upon.
This means, try to strengthen your elemental crits, poison damage, and mainhand damage if you are encountering issues with higher floors. Try not to worry so much about the heroic skill level of Orb.
I hope all this info helped... I'm not the best DQ player, so I may have made a mistake or two in my words... but if I did, I didn't realize it. I use a reactor build myself, and they are greatly satisfying. Keep on being you, good sir. And dream in neon.