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Andyman, I think that you will find if you can advance just the intake cam (on a DOHC engine) you will improve low-end torque and fuel economy. The engines you mentioned IIRC, being too lazy to check, are all single-cam engines. You are only advancing/retarding the timing of the entire valvetrain events relative to the crankshaft. By advancing/retarding the cams independently in a DOHC engine you are effectively changing the LSA. The 2008 (or maybe 2009) Dodge Hemi does this using a cam-within-cam arrangement to adjust intake timing by shifting the lobe separation angle, and improved both power output and fuel economy.
I've actually experienced what mechman600 is describing in a Honda v-twin air-cooled industrial engine. Those engines have to have the valve lash adjusted rather frequently upon initial break-in. So much so that out of the box they are often ZERO to .002" both intake/exhaust so that they wear in to their specification. If they get wider than spec by more than .004 or so the idle speed of the engine will begin to creep up dramatically because the engine runs more efficiently at the low (<2k rpm on a 3600 rpm governor) speed range. You're then left to adjust the carb's idle stop but even then sometimes the air bleed through the pilot circuit is enough to run elevated idle until you clamp back down on the valve lash.
I talked to an old-timer drag racer who used to adjust valve lash depending on how the car launched. If the car bogged off the line, he would loosen valve lash to get more torque. If it spun off the line, he would tighten it down to reduce low-end torque.
It's a trade-off between low speed torque and high speed power, but for fuel efficiency the high speed power situations are rarely experienced. The reduced valve lift at lower speeds should help improve combustion by increasing port velocity. If you're going to do a 12-valver I'm not sure if you're better off dropping 4 intakes or 4 exhausts...most 3 valve engines have 2 intakes and 1 exhaust. Ferrari once had a 40-valve V8 with 3 intakes and 2 exhausts. If i were to try anything that adventurous I'd probably drop from 16 to 8 and see if that was too much of a loss, and if I needed a mid-point I'd add the intakes back in. Reduced exhaust flow will increase retained EGR, but choked intake flow could really hurt your pumping losses.
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