If you have the ground clearance, try a bigger prop. I feel the ryobi does better with torque than higher rpm's.
I run an 18x10 on mine at about 6500 rpms. This is a basically a stock motor. I ran the numbers against your 16x10 and it's nearly 2lbs more thrust with the 18x10. An 18x8 would be worth a try as well. The APC props are leaps and bounds better than others I've tried in this size range. You will notice a difference. The added thrust , even 2lbs, can make a so-so plane a pleasure to fly.