Lots and lots of variables as to what RPM an engine is going to be most efficient at, but so far I've seen that lower is better... down to maybe 600-800 rpm above idle. My cars don't seem to appreciate "cruising" that low. And how low you can go depends on how much torque is available vs. the weight of the car vs. the speed vs. hills, etc. But, the Yaris would happily putter around at 1500 rpm in 5th at 35 mph, and the BMW will do the same on down to around 1200 rpm in 6th. (I can literally drive the BMW and use 1500 as a shift point)

The BMW will get 31 mpg at 80 mph all day long without giving it a thought. That's a little above 3,000 rpm. Maybe 3500ish. That's "good", but it invariably gets better as the speed goes down. I haven't experimented with it enough to say exactly how much. The improvement in mpg as speed decreases is partly due to less wind drag. For this engine, I think it adapts (variable cam timing, variable intake and variable valve lift!) to be very efficient regardless of engine speed.

But, I know I've made some fairly short trips (5 miles) from a semi-cold start and managed to recover up to about 32 mpg for that trip keeping the speed between 35 and 45... but that's using a lot of DFCO coasting. What will the car do just cruising at that speed? I just don't know.

So, all that just to say that I'm not sure what rpm you should gear your car to be at for optimum economy. My gut still says you should keep the RPM as low as possible. If you can highway cruise (60 mph) at 1800 rpm, do it!

Regarding the cam, more lift is good for overall efficiency. More air in... but then more fuel... but ultimately more power per cycle. I'd think overlap would be bad for efficiency. A lot of unburnt mixture goes right out the tailpipe with high-overlap.

As you're building more power, the car should be even more efficient at really low throttle and lower rpm. If your car is "struggling" to keep up at 1800 rpm at 60 mph now, adding more cam lift should give it more torque and make it happier. So, if you're going to do the cam, I'd do that first and see how it goes. Letting the engine rev higher than it is now AND letting more air into it (which is going to draw more fuel), can't result in better economy.

In general, I'd say that lower revs are good for economy. Higher low-end torque lets you get away with lower revs.