No need for a tune, there's nothing to adjust physically or electronically so theres no point.
I agree with Bennie's point that fuel economy isn't necessarily related to the health of the engine block and its components.
Oil consumption just means the rings might be a bit worn, so some is leaking into the combustion chamber and been used up in that process. Worn rings don't always equal extra fuel consumption though. I would try a heavier oil, like HPR 10 and compare the oil consumption.
The only real alternative is oil leaks. . I'd look at the engine crossmember, the valve covers and timing belt cover to make sure there aren't any leaks stains. Like you i go through about 1-1.5 ltrs every 5,000 and my car doesn't leave a bloody drop on the driveway, but it sure comes out of the timing belt cover when its on the road.
Servicing doesn't mean that all the consumables on the car and in good condition. Especially if you take it to the same place and they just do the regular oil/oil filter and air filter type service without actually check things like spark plug condition, fuel filter condition and other things that get replaced at much longer intervals like every 50,000kms.
If it was my car i would immediately put a fresh oil filter in and try a heavier oil. It won't hurt your car because its designed for motors with 200,000+ kms.
I'd put a fresh set of spark plugs in, air filter, fuel filter and use a fuel injector cleaner and a can of upper engine cleaner. The UEC won't do much on your car though, long trips like you are doing tend to keep the cumbustion chamber clean anyway. Its more short trips to the shop down the road every day that the car runs rich at warm up and builds up carbon, varnish etc.
Roof racks, driving lights and any other external adds on will add noticeably to your fuel consumption. So will tyre pressures, i have mine on 40PSI which is a bit higher than recommended.
Also sensors like the throttle position sensor, idle-air control valve and maybe some other things may not be functioning correctly and they don't always cause a check engine light. Oh that reminds me about the O2 sensors as well! Again they don't always show a CEL when they're not working. I just replaced my IACV and it caused no light, my TPS is malfunctioning and again i have no CEL.
The point is there's a huge long list of things that effect fuel consumption, and even assuming that your engine is worn (i think its just worn in personally), i don't think it accounts for all of the lost efficiency. Swapping a fresh engine won't address many, if any of thoseproblems and hence potentially won't do a whole lot to improve efficiency.
After all combustion is just fuel, air and heat and there's a list of things that regulate those components of the process. Its a pretty short list and there are methods to test most of them are working the way they should.
