I have used H and X cross overs and I can't say that I can tell any difference in power from no cross over. But, they have made a large difference in the sound. They have reduced the volume of the sound across the entire rpm range and they have smoothed out the sound and lowered the frequency.
In order to get any significant power increase they have to be placed properly. If you just put it where it fits it's likely not where it needs to be for max power.