I agree 450 mg sounds reasonable. It's less effective proportionately/relatively after 300–400 mg.

Your body doesn't compensate (via gene expression, regulation of receptors and via autoreceptors) to the same extent for blockage of (different) brain transmission/receptors. So it could be that after say, 400 mg, any increase only makes you more tired but doesn't help you any other way.

Taking too little may have a similar effect, but for a different reason, mainly.

The therapeutic range is 300–400. 600 mg may work, but the 200 mg difference is not similar to the 100 mg difference: the difference in effects of the 100 mg therapeutic range is greatest.

If you need anything above 400 mg to function, you might add a typical antipsychotic. It'll make you less tired and is likely even better than squeezing the last bit of efficacy out of quetiapine, causing an imbalance.