I would recommend using a 6-lamp T-8 fluorescent high-bay fixture for your shops that are 16' and higher ceilings as long as they are heated. They will use half the power as a 400 watt matal halide and put out the same light output.
The T8 florescent is the ordinary replacement for the old style T12. The old T12 is the old "fat" style lamp used with the "heavy" magnetic ballast, the T8 is the new skinny lamps used with the newer "lightweight" electronic ballast that now replace the old T12. The lamp you are referring to is called a T5. They are really skinny and are actually a little shorter so you cannot accidently install them in a regular flourescent fixture. They are basically the replacement for the metal halide fixtures. Are kind of spendy but do work well. They also turn on right away...they take a few minutes to warm up and put out full light but will give plenty of light to move around right away. Also metal halides loose there intensity over time, after about 5 years you only get about half of the light they put out new. The T5's will maintain most of their light over time and the T5's use less than half of the power of the metal halide. We install a lot of these as replacements.