There's always some local maximum most efficient process. If you take into account process inputs trees would likely come out on top. A lot of carbon capture tech can capture lots of carbon but consumes a huge amount of power/energy intense resources. My unit would be kg / (W * m^2 * s)

IIRC, bamboo ends up as "less wasted" when you turn it into charcoal than most woods. And you'd need to do a lot more energy-intensive processing of the tree to get it into a for suitable for turning into coal. With bamboo, it's mostly "cut into suitable length". With wood, it's at least "cut into suitable length", but may also require splitting.

But, yes, my main reason for thinking bamboo is that it is pretty fast-growing under the right conditions.

And with a Watt actually being "kg * m^2 / s^3", your unit ends up being s^2 / m^4, which dimensionally looks less pleasant.

Intensive cropping of bamboo for carbon capture sounds like it would cause as many ecological problems as it solves.

Not necessarily. Well, not necessarily more than doing the same with trees instead of bamboo.

But, yes, it'd require some care.