Note I'm not claiming the filtration theory is true or false - I also have doubts about it - I'm just saying your rejection of it based on 'we don't see such effects because of brain damage ergo the theory doesn't hold' isn't exactly sound. Actually I'm not sure you can use brain damage to prove anything here. It's also too general of a term for me. As far as I'm concerned a serious head trauma where part of the brain is removed as well as a lesion applied locally for the purpose of neurophysiological research are both brain damage.
If the hypothesis was true, we would expect the effects of brain damage to reliably create a richer and more psychedelic experience for the sufferer.
Why 'reliably'? Not all brain damage is the same. Maybe I'm misunderstanding the theory but I don't think it argues that, nor is it what happens in reality.
Also, why at all? Why would certain brain damage not be able to create a less rich experience? The theory doesn't say that is impossible I think?