its probably best that u stay away from any type of 'charger' thats not currently used or approved to work.
the ebay one was basically a hairdryer, stay away.
i know we all want more power, and an electric supercharger doesn't quite spring to mind 1st. seeing as u mention carby, maybe try a weber 32/35 carby. it'll improve throttle response heaps, and some bit extra power as a bonus. there are also mods for it to stop flooding and starving, but i've not had a prob with mine since i got it (only my motor cant hadle it.)
coupled with extractors/headers, and a good flowing exhaust should be right up there for under $500(aud). depending on where u are and how you want it.
My opinion, and seeing as i'm always in mud or water, electrics seem to be the very last item on my list for mods. and think about it, if it doesnt work properly (e-s/c) it'll only get in ur way and hinder air flow.
hope this helps.
Steve