Easiest way to test on your PC would be to unplug everything from your phone line and plug in a telephone extension lead to upstairs to see what speed you get.
I did this permanently as I did not want a homeplug and I still get the full 40 meg download and 10 upload and even ping times are exactly the same with my pc In The test socket downstairs as it is using a £2 10 meter phone extension upstairs.
I did this permanently as I did not want a homeplug and I still get the full 40 meg download and 10 upload and even ping times are exactly the same with my pc In The test socket downstairs as it is using a £2 10 meter phone extension upstairs.