I currently experiment with an affordable L1 GPS RTK system based on u-blox NEO-M8T with RTKLIB and higher quality patch antenna (with ground plane). I wonder what is the role of L2 signal in an RTK system (since the expensive RTK setups seem to use L1+L2) - how much precision/accuracy will it add to the system?
Yes, using the L2 frequency and the L2C signal can improve accuracy since it provides additional information and redundancies. The difference between the L1 and L2 frequencies provide information on irregularities in the ionosphere, while the added information in the L2C signal can help resolve ambiguities quicker and can improve reliability of the ranges to a particular satellite.
If you can get a certain level of accuracy at a certain level of speed, including the L2 information in the calculations can make it possible to derive more accurate data positions at the same speed, or similar levels of accuracy at higher collection rates.