I'm getting very poor download speed (~11 Mbps) in one part of my house, and I'm interested in replacing my current powerline adapters (AV500s) with something that utilizes AV2 protocol. My question is: how can I determine roughly how much improvement I may expect?

I'll supply whatever information is useful if I can; just tell me what you need to know to give a better answer.

Here are a few possibly-relevant facts:

Download speed is about 150 Mbps by cable and 32 Mbps with the AV500's on two different circuits in adjoining rooms. The 11 Mbps figure is between two circuits at opposite ends of the house.

Upload speed is pretty constant at 5.6 to 5.8 Mbps under all conditions.

All of the circuits in these tests are grounded (although some other circuits are not). The electrical box is less than two years old, but the rest of the wiring is over 40 years old.