You're correct, of course, that comments left by customers are statistically skewed towards the negative, and the relatively small number of people travelling in Thomas Cook Premium class makes the forums even more unreliable, but you should be wary of comments such as these:
I sat in the Premium Cabin on a long haul flight to Canada and the seats were appalling and did not justify extra expenditure. It was like sitting on a park bench and thankfully they supplied a blanket which I needed to sit on
We had upgraded to Premium on this flight to Kerala (10 hours) and the product does not compare favourably with the First Choice upgrade, particularly with regard to the comfort of the seat. We would repeat a previous comment that it felt like sitting on a wooden bench - had to stand up at regular intervals to get the feeling back in our legs! Would avoid this airline (even upgraded seats) unless there is no option.
I'm a little puzzled why someone who is normally happy (and able) to pay for a product such as World Traveller Plus would even consider booking a charter airline just to save a few hundred pounds.
But, whatever your experience, please don't forget to relate them here and on the other site (are we allowed to mention it by name?) so that others may be helped to make the right choice in future.