On an 80GB system partition, getting rid of the virtual HDD for xp mode makes a pretty significant change in the available space, and more free space on a HDD or SSD for that matter will mean more room for windows to scatter critical system files all over the place like it usually does before the drive becomes hopelessly fragmented. So yes and no, getting rid of XP mode and the virtual drive it created will not make your system faster, but it will prolong the honeymoon before the inevitable slow down.