If you want to be truly anal about it, yes it will become marginally less power efficient because the longer the wire, the more electrical resistance there is so more energy will be lost as heat in the cable.
But that is truly nitpicking, there would be no noticeable difference. I would even say it would be an immeasurable difference if it weren't for ridiculously sensitive ohmmeters.
I use a sleeved 24pin extension myself, only issue I'v had is that now I'v got 28cm of extra cable when I only needed 2cm to make the connection. Looks good, the Bitfenix Alchemy cables.