Is burning wood considered sustainable if the efficiency can be improved up to 90% compared to the 50% of conventional burning stoves?
Yes. Moz's comment is correct. When burning wood, you are in essence just shortcutting the rot process. Which means that instead of fungi and bacteria and bugs getting lunch, you get warm. These critters have their place, so it's not a good idea to intercept all of the production of a given forest. For this reason a high efficiency stove can be more sustainable.
I heat largely with wood, using about 4 cords a year in a low efficiency wood range, and a medium efficiency wood heater. I have 15 acres of forest. Doing some rough calculations I'm using the net production of about 4 acres, so I'm using roughly 1/4 of the available harvestable energy.
(Figuring net production: An aspen forest reaches climax conditions in aobut 50 years in Alberta's climate. Doing a wood survey on that forest shows something like 50 to 100 tons of wood per acre, ignoring branches smaller than 2". So that forest produces about a ton of wood per year per acre. In actual fact the biomass increase rate is fastest when the trees are between 1 and 2" in diameter, so by harvesting some of a grove, then I will be increasing the net productivity 10 years down the line.)
For achieving an efficiency of 90 % for a stove you need a very sophisticated technology.
You need at least a lambda measurement and a controlled air supply and a computer to regulate the burning process very carefully. Then it's possible to achieve an oxygen amount of 9 % and a temperature of 180 °C. This exhaust composition results in an efficiency of 90 %
But sorry, that's not a rocket stove.
Normally the average amount of oxygen in the exhaust of a good, modern oven is about 13 %. Less oxygen results in the production of CO (dangerous compound). Without a sophisticated technology and with a normal oven, that gets his amount of air by a natural way from low pressure inside the hot chimney it's possible to have an efficiency of about 70%.
For the described high tech stove you an expert for maintenace cause he has to check your stove more often to make sure that the technology is functioning well to do no harm to yourself or your neighbourhood. To use wood in a modern but "normal" stove means that you increase the efficiency from 50 to 70-75 %. So this approach is certainly a good and environmental friendly way to heat your home.
The efficiency of an oven depends on the temperature of the exhaus and the amount of oxygen. You can calculate the efficiency of a stove yourself here. http://www.normatherm.com/Abgasberechnung.php This tool is in german but essentially self explaining.