Air data rate nRF24L01
Increasing stability will increase the (usable) range. At least a bit. You could try decreasing the packet size, to make it even more reliable. There is no way to increase the output-power (only decrease). 

You could try different channels, and see which one gives you the best range in your application. 

After that you have to get NRF24 modules, with external antennas and amplifiers.