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Well:
1) Needs to be an efficient supercharger. Probably not a straight lobe, roots blower here... Probably Lysholm screw or a centrifugal.
2) Needs to be intercooled.
3) Moderate levels of boost. Two compressors running a low pressure ratios will be more efficient than one at a higher ratio.
4) Higher gear ratios to make use of the higher torque at low RPMs.
On a related note, I was reading through the latest SAE magazine last night and it had a long article about supercharging/turbocharging to improve power and efficiency. While most are going to turbos, both VW/Audi and Mercedes are going with twincharged engines (both engine driven supercharger and a turbocharger). The VW 1.4 TSI is already available (except in the US) and is a 1.4L 4 cylinder that makes 177ft-lbs of torque at 1500 rpm. That makes it pretty easy to keep the revs way down at highway speeds.
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