Steel yourself, I am about to go on an over-exaggerated rant.
I love this package and I think it's very useful, but I've got to be frank:
Your FlxButtonPlus class is completely useless.
Up one side and down the other it is poorly thought out as an extension to the FlxButton class. One reason is because it's not
an extension of FlxButton, it's an extension of FlxGroup, which is basically kind of counter intuitive by itself. There is also no down state. Why is there no down state? And then of course there's the whole thing where every button state has to have its own image, you can't just make one sprite sheet for all three states and have the Flixel engine split it up all in one nice neat package.
And lastly, you can't use the FlxDisplay class with it, because it's not a FlxSprite... it's a subclass of FlxGroup, thus making FlxDisplay useless if you want to center a button. What happened when you were making this class??
In all seriousness, I don't intend any offense with this rant, and I think that you've done a magnificent job with Flixel Power Tools. But you should seriously reconsider reworking your FlxButtonPlus class, as it contains some major logical fallacies in its design. You are completely welcome to tear my argument apart though, I'd like to be proven wrong.