I've done some lapping at Shannonville and Mosport DDT, but never in a DSM. I've done Mosport DDT in a Formula 2000 Car for one of their driving course which was ridiculously fun. My leg was shaking from adrenaline for half an hour after. Otherwise mostly in my former Miata. Hoping to get out to some events in the Bimmer this year.

I agree with Saloman that you shouldn't feel the need to optimize anything before you go to an event. You can have a great time in a stock shitbox as long as you don't have too much ego about getting passed by a better driver in an even slower shitbox

FYI there are autox events on the Mosport DDT once or twice a year. That's a great way to ease into it without spending much money or abusing the car too much.