Tested. Nightmare Fuel on the reworked Sealed Ahamkara Grasps, seems to be a 35% buff.

Quick testing on some ads at the start of GotD.

Using Midnight Coup with no additional buffs. Got a powered melee kill and swapped to Midnight Coup.

Normally hits for 12,796 per crit

Nightmare Fuel hits for 17,274 per crit

That’s about a 34.995% increase.

Nightmare fuel lasts for about 5 seconds.

Pretty significant damage buff, and this is before using surge mods, radiant and perks like bait-n-switch.

Juggling the weapons might get a bit tricky but a near constant 35% buff is pretty nice.