A custom UI cannot cause a memory leak, it's all just XML and image files. You can try
/force_low_mem_mode true
if you have memory issues, although the game should do that automatically once you run out.
I find it very hard to believe that previously two clients would only use 600 MB total on your system, ie 300 MB per client. For most people the game has always been using far above 1 GB and depending on texture quality up to 2 GB or more, causing crashes (hence the recent changes). I don't think you could ever get it down to 300 MB even using extreme performance mode and lowest quality textures.