I've got the z-eng. kit on my corrado which produced 274 bhp on the rolling road so it does give a genuine 70-80 hp increase for £2k but its all at mid-high revs power. The z-kit is very simple to fit as there are no oil lines, the vf-kit a bit more involved but ultimately more upgradeable. Not too sure on turbos but they seem to be popular in the US but not as widely available in the UK. The supercharger kit is bolt-on (cheap and easy to fit), produces reliable power and can be removed easily which appealed to me.