Evo II - Up to preference as you may employ Overshields or have +700 Energy Warframes
Evo III - Swift Deliverance for increased Projectile Speed which affects both Forms
Evo IV - Fatal Affliction for the Status Damage Stacking as its basically pseudo [Galvanized Shot]

For Corpus swap [Primed Heated Charge] for [Pathogen Rounds], And [Frostbite] with [Hornet Strike]

[Hornet Strike] can also be swapped out for [Scorch] for more Status Chance or [Gunslinger] for even more Fire Rate. If you go this route you spend 1 less Forma for Hornet Strike.

Arcane Options:
Secondary Merciless - Easier to maintain buffs up due to the erratic movement of the projectiles. Also gives additional Reload Speed which speeds up Incarnon Transformation as well.
Secondary Deadhead - More Recoil Reduction if you have issues with getting headshots in. The Incarnon Mode is more likely to get headshots with the main projectile as the extra projectiles aim randomly.
Cascadia Flare - Hits from Normal mode will still refresh the Buff if you miss a headshot.

Exilus Options:
Lethal Momentum - Additional Projectile Speed for both Modes if you can handle the kickback.
Steady Hands - Reduces Recoil making aiming easier but projectile will still have some delay.