Something that i'd like to point out in regards to the whole "mountains of work" thing:
I get that going back and changing hardcoded systems is a pain, backwards compatibility is important, balance between game modes, etc.
But heres the perspective I look at osu!mania from. I'm a 4key player, not an osu! player, I will play the best 4key client made available to me with no real attachment to one just off of name alone. Whichever client provides the best suite of features will be what I elect to go with, end of story. The other two clients i play 4key on both provide 4key specific leaderboards whereas osu!mania does not. And regardless of the reason why osu! is unable to provide this at the moment, the fact of the matter is that it doesn't. Ultimately, as a player, I'm going to go where the best support for my gamemode is. I'm not interested in excuses. If 4key isn't important enough to the osu! project to warrant all that work, its understandable. But in the meantime, I'm going to play a client that does what I'm looking for, not osu!mania in the hopes that someday it'll be able to stand up to the competition.
Not that im under any illusion that my individual player-ship will make or break osu!, just my perspective on the matter. As stated above, a simple leaderboard filter would be absolutely fantastic if thats a realistic balance between workload and results.
EDIT: And beyond leaderboards, osu!mania does very little (aside from multiplayer) that other clients do not while being far more limited in a variety of other areas. The leaderboards alone are not what causes me to take this stance, however they are a very relevant factor.