I'm having some issues understanding properly how the loading of leaderboard scores actually work.
If I specify a range of the best global scores of 1-25, if the local player is at rank 50, will it still be loaded and be available with leaderboard.localPlayerScore?
Also, are those scores the only scores that are actually loaded onto the device, and the filtering happens on the server, or does the GameKit API actually load everything, and then filters it on the device?
Yes, the localPlayerScore attribute of the GKLeaderboard will be available after loading, regardless of the player's actual rank.
The score filtering happens on server side, as loading every score would be very wasteful. This generally applies to nearly every application that handles network data (unless it relies on offline usage).