I recently replaced my [central, ducted] heat pump, and the new one is rated with a 4.981 SCOP. It's fairly efficient, but not the most efficient on the market.
It would be nice to actually link the source[1] (PDF).
The stat is that you lose between 25-40% efficiency for "typical" existing installations. The document goes on to explain that an insulated duct which doesn't leak doesn't have this kind of efficiency loss.
