These seats are used on regional and suburban trains elsewhere in Europe (they must be laughing at us for using them on long-distance trains). I wonder how they hold up there. The seat cushion specification though is, I understand, unique to the UK, so I wonder if that's part of the issue.
Dutch railways (NS) use the same Fainsa Sophia seats on their newest fleet of short-distance commuter trains, the Sprinter Next Generation (CAF Civity). Dutch railways held a seat competition a few years ago, they had three distinct types of seats on display and asked the public which seat they preferred. Apparently the Sophia won.
Since their introduction, many Dutch commuters have been pretty vocal about severely disliking the seats. "Hard, uncomfortable, and forcing you to sit in a certain way".
I've sat on them myself and while the seat is okay for a 20 minute journey, I can't imagine the same seat being deemed suitable for a 4hr journey on a long distance train - no matter how much padding you put on it. The shape of the seat itself just doesn't feel right for it.