I did the same as you on my G305: I've used that extension cable that came with the mouse and put the dongle on my table in front of the mouse pad. I don't know what else you can do to help the mouse.
About checking how good the wireless works, I tried putting my phone playing Youtube next to the mouse while I experimented with the "Mouse Tester" tool. The connection still looked fine, the mouse could still send updates at 1000Hz. I don't know how else to test for wireless problems.