10-20hp? that is a little optimistic my 3sge must be about 200hp now from those estimates
besides it doesn't "use" horsepower when it is off it is just an idler pulley at that point the only thing you are saving is weight and in a road car it won't be a noticible difference unless you decide to do some more serious gutting out.
When it is on it saps a bit of power but why would you be street racing with the a/c at full blast? If you are that worried just flick the switch to unload it off the engine