I can't quite agree with Brendanuhs. If I take your point, then for some people HD on a 180 bpm, AR 3 map may be pretty easy, and DT on a 100 bpm map could be difficult. If you talk like that then EVERYTHING is relative to the player. This just means that it'd be impossible to set any sort of score multipliers because it wouldn't 100% completely fair.
I find that the current system is alright. HR in general isn't too difficult. On an insane map, it would be thought that HR would be too much a big difference. FL I would say if comparatively more difficult, however, to some people it can be a very easy mod.
It's not possible to have a completely fair system of score multipliers if you think of things like that.
This is all just my opinion, if you hadn't had gathered.