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It is interesting that result that you have got with air pickup changes.
Could some of effects have to do with angles of intake piping? I did read some time ago from results done with pipe and different angles in it, there was more loss of velocity, more there was angle, even small angles seemed to cause some effect, so that is something what comes to my mind that can also be part of it.
Problem I have with intake pipe and pulse tuning is that it would be at least XXth resonance so far away (it is Hz of intake valve opening/closing and distance comes from speed of sound, basic idea is that pressure wave goes back and forth in intake runner and one tries to match dimension of runners for Nth pulse so that pulse is at intake valve when intake valve opens), with direction changes, with restriction valve (throttle butterfly) I doubt that there will be anything left from pulse. Without any turns or restrictions there could be more chance for that effect, that is why I'm skeptical and think that there can be some other feature that causes improvement.
But there was ram air topic where I posted link to test and results to pipe angles somewhere in here.
If one has kind of block exhaust manifold, then there can be of course lot of odd effects happening with larger exhaust.
My recipe for economy tuning would be to get exhaust manifold that is tuned to max torque rpm, then swap silencer boxes, keep diameter of pipe standard, if one is really adventurer, new individual runner intake manifold with runner length matched for max torque rpm could be put in, but that is expensive one, however anything japanese and relatively modern, it should already have that (some even have good exhaust manifolds as standard), performance models might have intake tuned to max power however, some might have adjustable intake runner length.
Easiest first, silencer boxes to something more flowing, but still tastefully silent.
Chart posted earlier in my opinion can be perfectly good guide as especially with gasoline burners you are not moving that much of exhaust or air when cruising speeds are used, if you think that on highway car is using 20hp, rpms are relatively low, you don't really need much of diameter, with no restrictions from silencers it would spit out gases really well and more efficiently which leads even less exhaust gases.
There is difference of exhaust manifold and exhaust pipe size, exhaust manifold is critical in length and diameter, but I consider exhaust pipe to be such that exhaust manifold just carries evenly organised pulses out and as long as there is no restriction from rest of pipe thing works well, that is why I would attack first to those silencers.
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