Following 3.0.3, Nintendo released several minor stability updates before shifting focus toward future hardware: Ver. 3.0.4 (May 13, 2025) : Fixed a synchronization issue on the 3DS Music Park track and a coin-disappearing glitch on N64 Rainbow Road Ver. 3.0.5 (May 21, 2025) : Addressed an issue preventing the upload of Time Trial ghost data for 3DS Music Park.
While specific details about version 3.0.3 might not be widely documented due to the evolving nature of software updates, typically, updates for games like Mario Kart 8 Deluxe focus on: mariokart8deluxeupdate303nsprar better
If you are looking for improved performance (the "better" part of your query), it likely stems from the Version 3.0.4 Following 3