I like CP, but I would not book it is I was in a rush. This is a BIG restaurant, andpart of what you pay for is the character interaction. I think 30 minutes would be pushing it for a CS restarant, never mind a TS character breakfast. Part of your problem is that like the rest of WDW characte breakfasts, they tend to run behind. If you check in 10 minutes ahead, that will not guarantee you get in on time, so jut waiting to be seated could eat up part of yoru time budget.
i have been reading reorts that the mine train is not a given with early park ADR's so while I hope you can make it, And I doubt that this will occur with an 8:15 ADR, please make sure you understand that if you get a front space in line, it is a Pixie Dust moment.
To help gain your early exit from the restaurant, make sure you get to the park EARLY> THese restaurants do not go in order of ti, they look more at check in to get guests seated. ALso, pay your bill upfront so you can leave when you are ready to go, and do not need to wait fo your very busy server to reappear.
Good Luck!