My best comparison is with Jason M0NYW who is using the same G3XBM transverter, but I believe a more conventional antenna. He is also reporting his ERP as 10mW. Jason gets better results that me. In the last 12 hours he was spotted by 39 stations, whereas I was spotted by 16.
How much difference is there? Clearly, he is getting better results, so his antenna and ground are better. Most of my reports are quite close to the limit. Also, for some stations, the local noise can put a stop to very weak signals being decoded. When near the limit of decode, a few dB could make a great deal of difference.
So, it is very hard to say how much better Jason's system is than mine.
All I can say is a more conventional setup is better. What I can say is the earth-electrode "antenna" in the ground does work and is just about invisible.