Where, Efficient = minimum resources, maximum repetitive strain injury.
The bad news is that to get to level 70, you will need to craft 22,288 shards.
The good news is that you will only need approximately 6900 lesser shards in the appropriate school.
All your experience will come from deconstructing shards.
To test this I built 200 'Greater Impact I' shards. Total cost 200 Siberys and 800 Lesser Arcane.
I then deconstructed them; got 200 XP, 404 Lesser and 67 Greater. Then I turned the greater into lessers, leaving me with a total of 739 Lessers.
So, I got 200XP for 61 Lessers. Keep doing this, 22,288 times, and you will hit level 70 for about 1/3rd the normal cost.
Serious Bit
I think this just illustrates another reason why the returns should be based on the actual inputs of a recipe. This should apply at both low and high levels.
If I put 4 Lessers and 0 Greaters into something I should get 1-2 lessers and ZERO greaters back.
If I put 80 Lessers and 10 greaters into something, I should get (say) 24 Lessers and 3 greaters (30%) back.