Try a 18 x 6, 18 x 6W or 18 x 6-10 prop. Experiment with different makes of props.

The 16 x series props develops only around 15 lbs thrust, the 18 series punches out 24 pounds. But, to get approximately the same RPMs, when going to the next largest prop diameter, you must lower the pitch by 1.

Thus, to keep the same RPMS as your 16 X 8 prop, use:

17 x 7

or

18 x 6

This is rule of thumb at it's best. You will see a slight decrease in RPMs simply because the increased weight of the bigger prop will induce a load. That is, unless you want to invest in the Mezlik prop, or other CF variant.