I have used these in the past (excuse long url):-

http://www.ebay.co.uk/itm/2-x-CAT5e-RJ45-Network-Cab...

As well as the coolport products with good sucess rates.

They are handy as get you out of trouble when there is no time to run another cable, but would never use them long term.

Why not just run another cable back from the router to the home plug at the wall socket?
Or try the homeplug on the extension and see how the speeds hold up? The newer ones seem to cope better with 4 way strips