On the subject of of would it be better to just grow the tree and burn the wood, NO not really.
First if we could make it produce power while growing we would have that power, what ever it be? Then we burn the wood once the tree has matured to the point we want it replaced. In turn we have 2 sources of power 
That would be getting the best of both worlds. And also those not wanting to burn wood could bennifit from growing the trees for power, then sell the wood to those that do want to burn it.
As for the effiency of burning wood, one must remember that many usefull items are tossed in the trash or otherwise waisted. For instance you can collect the wood ash most people throw away, filter water through it, and the result is LYE you could use for making soaps or red devil drain cleaner! Just a little more detailed than that, but still simple to do.
Most people, including me toss the wood ash normally thus losing a useable product. Also the Ash could be used for cinderblocks, mixed with concrete for a filler etc.. for low strength applications. Again, we toss the usable ash thus making the burning of wood less effiecent than it actually could be.
Same with creosote, it could be used for many things, treating wood fence posts or telephone poles, but we toss it! Since it is the main cause of chiminey fires obviously it is flamable itself and could be burnt in some way to produce heat.
Wood buring could be 95% effiecent if we would go to the effort to make it so, but we do not. We take the easy way out, burn lots of wood, let hot heat out the chiminey, trash the ash and creosote, and that is why wood is not considered all that efficeint of heating source.
