I would ask your psychiatrist about Abilify. This is the newest antipsychotic and least likely to cause sedation and weight gain. Psychiatrists sometimes use antipsychotics as mood stabilizers and it depends on the person and what they have (some people have bipolar with psychotic features) so I can't make a reccomendation there but among mood stabilizers, Lamictal is the most tolerated. And there are medications out there in development (I'm on glycine which is in Phase II study by the FDA) that don't cause sedation, weight gain/diabetes or tardive dyskanesia so as new treatments are approved people will see better results with less side effects.
400 mg is a lot of seroquel. I have been taking it for 3 months and I am just now getting to where I can take 100 to 150 mg at night and not feel groggy most of the next day. Make sure you remember to not drink alcohol with seroquel or it will intensify the tired feeling.
I can't second guess your psychiatrist. Some doctors have their own experience base and use medications like this because they judge it to be effect in a particular case. That said, you should not be taking any medication that makes you feel doped or drugged or too sleepy, so you should call your doctor, tell him about your experience with the drug and ask him to either modify the dose or the medication.