In your practice is it consistent to see huge differences in personality in patients on and off their psychiatric meds?

NO. Consistently, no. Sometimes, yes. It depends substantially on the diagnosis, types of medications, and many other factors. For instance, in some patients with adhd, they are angry, short tempered, and belligerent initially, but when they are taking a therapeutic dose of medication, they are much calmer and focused. If they stop their meds, the old symptoms return.
That depends. That is variable. It depends on the condition, the treatment and the person. Sometimes a change in behavior, mood or thoughts is noted rather than a major change in personality.
Psych meds. Psychiatric meds given for the right diagnosis and if they work appropriately without major side effects are designed to improve symptoms and behaviors, this may happen approx 50 % of the time, you see a response and then eventually hope for remission.