I don’t think it was the ND filter so much as the f stop. There’s a diffraction error that can occur at maximum iris closure that actaully reduces quality from a pinhole effect. That’s why movies are shot f/5.6 and have a shallow depth of field. That’s where the style came from!
Did it make everything look fuzzy? You make it sound like only parts of it look bad. You can try sharpen, antialias with a resizer instant hd, enhance color contrast, or just go all out for fill effects and shadows. If they want crispy clean, its a reshoot.
p.s. if you want, you could try dragging footage into my AE project and see if you like the color enhancement.
https://forums.creativecow.net/thread/2/954145