I think the problem with soft wood and carbon sequestration are possibly the same underlying problem. If they are still gathering sunlight and carbon the same

I think the problem with soft wood and carbon sequestration are possibly the same underlying problem. If they are still gathering sunlight and carbon the same way but gaining size quicker then there is just less carbon per cubic cm. There'll be some advantage in being able to get at more light and carbon sooner, but as carbon stored per square meter of land it's probably less. That is not to say that you could not in principle make trees better, but I think the changes would have to be something fundamental to the cell physiology. A C4 carbon fixation pine tree might be better but it might be easier to turn bamboo into a pine tree than it would be to make a pine tree work with C4. Evolution is really good at balance, tweaking a bit can knock a lot off balance. It's not an impossible feat to make something better but it isn't easy.