Retia mirablia occur in all lamnid sharks. Retia also occur in several non-lamnids. Not all sharks that have retia mirablia actively thermoregulate. This behaviour may also be an adaptation to conserve energy while permiting searching a wide range of depth in the water column. Since most enzymes are clunky, unstable, and very heat-sensitive molecules, it seems likely that dumping excess heat may be a problem for these sharks in warm environments.
Most hormones can function only within very narrow ranges of temperature.
Thus large, endothermic sharks in warm water may have a much tougher time coping with the problems of overheating than do cetaceans. In mammals, the larger the animal, the lower the relative metabolic rate. I hope at least some of the foregoing is useful or of interest.