Yeah, you need to lighten mid-tones and sharpen if you use some of the darker/marvel comics, but ps does a good job of that.
I noticed that canti/mangle produces a strange banding effect in the backgrounds of images that is quite annoying to me anyways. Also mangle always sets itself to kindle 3 and won't save its preferences so I just deleted it and use PS instead. The PDF reader does it on the kindle too for some reason. I like to see plain white or w/e.
But using Photoshop works good, you just have to get it set up and then it takes like 2 seconds to do your conversions. Touch ups aren't too hard to do either.