10% damage is actually pretty huge. Corki went from OP to in the dumpster for numerous patches after they removed the free 10% damage on autos his passive had. It took like 5 buffs to make him good again.
Kalista already helped UoL win IEM Oakland, so she is competitively viable already, while still being trash for 90% of the playerbase. One of the main balance problems of Kalista is how strong she is in LCS/challenger play vs regular play. That is a huge buff (ok revert, but still) to give her that would probably make her pick/ban in LCS again, without making her viable in bronze-gold. IMO, they need to fix her passive (the infinite dashes part), rather than just give her more damage in her toxic kit.