Last Wednesday i had this situation in gnw.

My enemy field was made by one mb blitz team in top, one tendo mb team in front of me and one full pains supporting team in the lowest field. We reached round 5 with me only left alive (i was supported by two low levels, so was to be expected) and roshi was in the top line and sakura in the bottom line with han and azure fang in the middle line, so, due to power difference, i couldn't be killed by them, but both my roshi and sakura, as you can figure, were hit a ton of times per round. The point is that the damage my roshi landed in round 8 (the round where i managed to win) was not much higher than round 5 one, while, instead, should have been significantly higher due to 3 more rounds of hits endured, that's why came up to my mind a cap could be there, maybe to prevent situations like this one.

P.S. could be, as you said, that the cap is +400%, that makes sense with the +200% with roshi, since roshi's mystery benefits only for half of its damage by ninjutsu. About sakura, my sakura usually hits for 700-800 in round 1 against a comparable power enemy, but I've never seen her hitting that enemy for 4k+ and in bonds happen you get hit enough to make it. The max i saw, with no other boosts, was a 2500-3000 in those situations but is hard to make a sure assumption due to rng volatility.