If I get a jackson kvx10, change the pickups to emg 85x and 81x and change the tremolo with a better one would it sound like one of those high quality guitars?
Well depends what amp your playing through. Also those higher guitars have better quality wood too which does affect the sound, but most sound comes from your amp.
Like he said, it's all in your amp. If you're playing through a Marshall MG, or a Spider III (or any modelling amp for that matter) you're not going to get a high-quality sound.