Most of the lights that are designed to scatter light are actually quite a bit less efficient than some PAR38 and PAR30 bulbs.
The reason is that the "scatter" type lightbulbs use larger numbers of tiny LEDs, 1 watt or 1/3 watt (or anything in between). Typically, these are quite a bit less efficient than many of the 3-8 watt LED dies.
There's nothing magical about the 3-8 watt size LED, it's just that technology right now develops the most efficient LEDs you can get in this size (Cree XML for example, or XPG or XRE, all of which are more efficient that most of these tiny LED emitters).
Your idea of putting these in a close-fitting hood, using a reflector, is not a bad idea at all, but this can also be replicated using LED's with no optics at all. They put out (normally) a light spread that's around 120 degrees, which is quite wide.
My 11g tank is 36" long (and 9" high and wide), and it has a small hood that sits directly on the tank, and has 14 Cree XPG running at 500mA. No light spill at all, and no optics. Looks wonderful. It is, though, a DIY job and a bit more complicated than the edison-base option.
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