By outlet flow I assume you mean total flow on the cooling water side.
You’d have to look at how they calculated their COP, but likely on that front more cooling water flow will only mean the heat pump has more contact with water closer to the inlet temperature, so you probably get more capacity and therefore a better COP. However your system on the whole is now spending pump energy for that unknown and likely small benefit.
Similar answer to the whole question - if you flow more cooling water you can probably get more capacity, but you pay the price of needing more cooling water flow at an increased pressure drop. If you flow at too high of a velocity it will at some point erode the piping and coil internals.