Probably significant. If I were to make a SWAG, though, it would be the hard limits of dissolved oxygen on maximum metabolic output that would be the biggest s

Probably significant. If I were to make a SWAG, though, it would be the hard limits of dissolved oxygen on maximum metabolic output that would be the biggest showstopper. There is very little oxygen available in even the most oxygenated water compared to our atmosphere, and without a powerful oxidizing agent available to provide the chemical oomph required, there is only so much resource-hungry brain a gill-breather can support, no matter how useful the neural material would otherwise be.