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I believe I read somewhere years ago that the hemi combustion chamber was the worst for thermodynamic efficiency, due to high surface area. It also has to have a pop-up piston to get acceptable CR (as a hemisphere has greater volume than a wedge), and that popup impedes flame travel. I'd speculate that it therefore is less tolerable of lean mixtures, which are harder to burn.
Perhaps that's teh angle that Pale Roader was referring to?
It would be interesting to see BSFC of a 5.4 Ford, 5.3 and 6.0 LSX, and 5.7 hemi - as-is, without MDS or other features. And of 4.7 vs 5.2 Maggie.
So did we decide on the original 4.7 installation question??? 
honestly, the new hemi is a "hemi" in name only. it probably would be more accurate to call it a polyspheric or pentroof engine.
the dual plug config also addresses flame front issues created my the larger surface area.
there's no reason you couldn't do a 4 valve pushrod hemi, either using a forked rocker. you may end up increasing the ratio of valvetrain weight to valve surface area, though, because you have 2 valve stems
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