e.g. is there a specific alt for best speed and/or economy, high as you can? presumably at some point the lack of aerodynamic efficiency starts to hurt?
Depends on what your other constraints are for "economy". Are you prepared to fly as slowly as necessary to achieve the economy? Do you want to fly at a particular power? A particular IAS? A particular TAS?
For speed, the answer is still "it depends" but is easier to express. Speed at maximum power is generally highest at sea level, but at a particular power TAS is highest at the highest altitude at which the power can be achieved.