My psychiatrist offered me to try zoloft at 225, which is +25 above the maximum. But I refused since I was not better at 175 and 200 than at 150.
Another psychiatrist told me that most of the time, he is working "out the book", meaning that he uses above the max when he thinks it's appropriate. Information about overdose triggers with meds are kept secretly by doctors and pharmacists, to prevent giving information to people planning to end their life. If we take zoloft for example, an overdose usually not kill, but damage the brain and cause paralysis. But I've no idea about how much mg are needed. I doubt that doctors play around the overdose triggers, even if they may go sometimes above the maximum recommended. The consequences could be too big.
I've also met psychiatrists that refuse even to give the maximum recommended dosage. Judging this information also sensitive. And you cannot base your idea of a maximum with the pills. For example, seroquel xr stops to 400mg. But people can take up to 1000mg and even above sometimes (well I don't remember if it's about seroquel, or the xr version). So the last bigger pill gives no clue at all.
The max recommended is calculated with experiments and is a kind of discharge of responsibility for drug companies for people that use a higher dose.