As noted above, just make sure that the altimeter has a mach lockout and the switch band vents are not directly below the rivets (or other fasteners) and it will be fine. I have not pushed my DS Jr. over Mach (don't have the 6XL case or know anybody who does) but we've put the 54mm Darkstar 3 to Mach 1.2+ several times with no problems running the 54mm 6XL Imax. Stock build, no extra reinforcement.
If my sims are working correctly it looks like the 15 second maximum delay will not be long enough, and you will need to remove the motor deployment charge. In that case, I would recommend putting two e-matches into the apogee deployment charge, wired in parallel, so that if one match fails the other can still light the BP. You should test that your setup (altimeter, battery, and matches) will work with a second match wired in parallel, or find someone with the same equipment who can confirm that it does.
I used to not worry about this until I got a batch of matches that had an unusually high failure rate, and I got religion after my L3 cert rocket was saved twice (including the cert flight) by the backup apogee event. A fiberglass rocket can survive a botched main deployment, but if the apogee event fails and the main deploys during the ballistic descent, it's ugly. You wouldn't think that a parachute could scream but that's what it sounds like. And you'll lose a lot more than just the parachute. And it's just plain dangerous if there are humans in the vicinity.