I'd second that opinion. White LEDs (including Luxeons) generally run at a forward voltage somewhere between 3 and 4 volts (hence the large number of options for 3D and 4D Mag conversions). To run off a higher voltage, you need a buck regulator. Virtually all those I've seen are straight constant-current sources, with the excess voltage dropped across the regulator. If you use such a regulator with a Luxeon III at 750 mA, you'll be drawing about as much current as with the incandescent bulb (the LED would draw less power than the bulb, but the remainder would be dissippated as heat by the regulator).
One way of "tweaking" things is to run multiple LEDs in series - a "back burner" project I have (bike light - want low current drain for lightweight battery) involves 3 Luxeons in series running off a 12V battery (battery dictated by other criteria). If you were to run 3 Luxeons in series, you'd get the same current drain as for a single unit, but 3 times the light output.
18 volts is a bit of a "sour spot", since for packing efficiency 3 Luxeons and associated optics is good (3x Luxeon 1, especially with the "R" bins that are now becoming more common, gives good light output at 350 mA), and would "fit" a 12V or 14.4V power pack. You'd be looking at 4 Luxeons, which isn't quite as good from a geometric standpoint (and you can't get a quadruple optic, while triple optics are fairly common). If your drill were a 24V unit, that would fit nicely with 7x Luxeon I (treat each unit as a hexagon - 1 hexagon surrounded by 6 others).
On the other hand, if you could obtain a buck regulator that worked as a "charge pump" (i.e. draw less current at higher voltage from the battery than is run into the load), that should help extend battery life. How long does the incandescent last on a charge? My guess would be somewhat over 1 hour (750 mA bulb, drill packs are typically around 1 AH).
