That can certainly be true of plaice - which I often buy when they are still alive. I confess that I quite like it with plaice!! Gives a certain refinement to a fish that can otherwise be a bit ordinary. Never experienced it with brill or turbot. With Dovers they can be tough and rubbery when eaten on the day they are caught. Higher temp cooking seems to have a de-naturing effect to an extent. Skate can be the same. Not convinced I’ve completely mastered things though as I suspect there are other variables at play.