Hmmm...let's see if I can help you with your timeline a bit.
Every time we've been to CM's, no matter what time of day or year, we have had at least a 15 minute wait after our ADR time. For character meals, I would allow 90 minutes to really enjoy the experience so that you can see all the characters. That's why you're there, right?
After breakfast, it will take you about 5 minutes to walk to the entrance of the Contemporary, 5 minutes to walk to the entrance of the MK, 5-10 minutes to get through security (if you have bags) and get through the turnstiles...
and it depends on when you want to arrive for RD. Do you want to be at the head of the pack? You'll need to be there at 8:30 or earlier. Do you just want get there to see the park opening ceremony right before 9:00? (Which is fun, by the way.)
So...your answer is: it depends. It depends on what kind of experience you want. IMHO, I'd rather schedule Chef M's for a day where I wasn't rushing around trying to hit a RD afterwards. Considering the heat that you will be experiencing during your vacation, I wouldn't want a heavy breakfast before hitting the park, either.
If it is a choice between 7:40 & 7:15, I'd keep the 7:15 am ADR and keep a very relaxed attitude about making RD.
Have a great trip!