I've got the YFZ450 calipers on my Banshee, and yes they fit right on. Just have to remove the backing plate/heat shield thing which is pointless anyways.
And I do think they make a difference, but I replaced me entire front braking system so it's hard to say what made the most difference. The reason for this change is that my stock master cylinder took a shit, and well Banshee stuff just sucks anyways..
I went with the 450R master cylinder, ASR steel braided brake lines, YFZ450 calipers, and EBC pads. And in my opinion, HUGE improvement over stock. I mainly ride MX and the difference is huge, not just in clamping power, but I get hardly any brakefade now, and it used to be unavoidable.
I say it's worth the time and money if you ride your bike hard like I do. If you're just a cruiser or drag racer it may not be so worthwhile though.