We've finished testing confirming that there is no evidence of a debuff cap in our version. Our testing showed that ATK down could be stacked up to -60% with full effect, and there's no reason to believe it couldn't be stacked to -70%.

Google Sheets: KamiPro Debuff Cap Test (by Dr. SlickFenix)

This testing was performed by the Godtamers Union Research Division, with the bulk of the work done by Dr. SlickFenix, and review and calculations performed by sanahtlig. Predicted damage differential was calculated using the Damage Calculator in Sanahtlig's Kamihime Project Toolbox.

Until the damage cap is implemented (2/8/17 in the DMM version), stacking ATK down will be incredibly potent, reducing damage taken by as much as 70%. Those running with Sol/Zephyr and Jormungand/Yatagarasu will enjoy a large advantage in Ragnaroks, provided they can stack these type C and eidolon debuffs with strong type A and type B ATK down debuffs.