We've already got super trees (genetically improved). These are selectively bred trees, not GMOs; stuff that gets super site-specific, like 'north side of a hill within 100ft of a stream'. IIRC Weyerhauser pioneered the work in this area...I want to say back in the 60s. They've reduced the time from planting to harvest substantially, something like ~80 years to ~30, and the new stuff is just growing faster.

The main problem is that the growth rings get huge, the lumber is technically stronger, but it's much denser and not well suited for all applications. If you look at old growth, you see sub-mm growth rings in Doug fir, and it's very light. Super trees can be quite substantial, like 1/8 to 1/4 inch. I've heard they burn hotter and more readily too. In my personal experience, old growth will burn all night in your stove but the younger stuff won't last half as long.

Been a while since I've poked my nose into forestry, I've probably got some of the details wrong but that's the gist of it anyway.