Gotta_gofast makes the best point here. I'm just gonna add a few things, Dont forget with a turbo setup, you need to rework your enine, different pistons,etc or you'll be goin to the scrap yard pretty fast. So there's a lot more that just 'slapping' on the equiptment and driving away.
In defence of the electric turbo though, the inefficiencies are only in the way you power it. Something to consider, electric motors are more efficient than gas. And yes it does take the ICE to power the alt to power the EM, however, it takes roughly 1.5HP ( ICE ) to generate enough electricity to run a 1HP EM. That same 1HP of Electric Motor is outputting the same torque,etc as what would take a 6HP ICE to produce. Proof of this can be found in electric vehicles, e.g. A ford Ranger when converted to electric only requires a 20 - 30HP EM to reach highway speeds.
That being said, if you were to compare the little amount of boost in each scenerio...say a 1LB boost in each, you would more than likely find the electric turbo to be more efficient.
Taking into consideration I dont know a lot about mechanical turbo's, I cant compare the how much power they actualy consume. But I can make an arguement for electric motors
All that being said, I'm not sure if personally I would waste my time with an electric turbo, BUT if you wanted to play with it just to see for yourself, I'd goto a hardware store and pickup a duct booster fan, they run around 10.00. I'd plum that into a toggle and go from there.
Something I just thaught of though, a turbo controls air flow via the rpm of the engine, where as an electric turbo w/o and electronics cannot do this, so you loose efficiency there.
To make an electric turbo more efficient, you could utilize one of those newer thermolectric generators that hookup to your exhaughst pipe, these produce significant amounts of power and dont draw anything from your engine...
Sorry if this is scattered, just trying to stay neutral and add onto what he said