We've seen with SpaceX's Merlin engines that the initial design had a lot of headroom for subsequent increases in chamber pressure and mass flow rate (from supercooling and densification) to gain increased thrust. Falcon 9 Block 5's payload capacity is almost what was initially estimated for Falcon Heavy!
Battery technology is improving quite rapidly as usage growth for transportation and electric grid applications drastically increase investment in R&D. This offers the opportunity for Rocket Lab to adopt batteries with improved specific energy or power. The obvious simple use of this is to simply swap in lighter batteries with the same energy and power ratings, gaining higher propellant and payload mass accordingly.
But can they do better, by increasing power or energy within the current battery mass envelope and driving the engines harder or longer? Drive the pumps faster to increase chamber pressure? Follow the same propellant supercooling-densification path as SpaceX with the increased potential mass flow?
If the data to answer the above isn't available, what information would we need about the engine's current specs to be able to figure it out?