A 810nm led has a 350mW/sr rating. How do you convert to mW/cm2?
350 mW/sr means \$(4\pi)(350\ {\rm mW})\$ into the complete sphere around the source (if the source were to be emitting equally in all directions, which of course it isn't).
So at a distance \$r\$ from the source, the peak irradiance is \$\frac{(4\pi)350\ {\rm mW}}{(4\pi)r^2}=\frac{350\ {\rm mW}}{r^2}\$.
As shown in figure 4 of the datasheet, you can indeed reduce the output irradiance by reducing the drive current: