I asked about this a while ago on one of the child seat safety pages and was told they aren't safe. I think we had ended up buying it anyway, but didn't use it because the car we hired for the long drive turned out to come with a Diono High Backed Booster. These are designed for children up to 160cm so he fit into it even though the normal ones don't accommodate taller children. If you wanted to get one of these instead and he's open to it, there is a Diono booster on sale at Smyths for about £30. DS1 said it was actually seriously comfy, and you could use it just for the long drive! It's cheaper than the headrest thing.
FWIW, that version of the product is a knock off one - we did the same thing! The original is called a Cardiff Headrest Support and is quite a bit more expensive. I don't know if the original one has crash testing, but I'm sure the knock off ones don't - and I think the concern was that they could affect the working of the headrest (which plays a safety function) in a crash.
I'm not hugely convinced that they are unsafe, FWIW, my thinking is that surely sitting with no booster at all provides 0 protection, so having something there that isn't designed to add protection isn't going to take away from that. I also think it could help the seatbelt stay in the correct position, since when DS1 fell asleep on a previous journey, he was lying over completely out of the seatbelt. It was late, and there wasn't much traffic so I didn't wake or move him, but I was uncomfortable with it.
We still have it and I am still on the fence about it. I might bring it out later in the year for a long drive we are thinking of making.