Are you stalking me!?
I made pretty much this exact post a few weeks ago..on a different forum!
I just switched to an M675 38/24T.
It fits fine with the BB that’s included in that set.
I noticed a big difference in ratios too.
Things to look out for:
1) Chain length, I removed a whole link as it was too long, 1 whole link is 3 teeth so going from a 27 to a 24 makes sense.
2) Front mech height. The direct mount front mech wont really go low enough and I am experiencing some chain rub and the extremes of the gearing. I have borrowed a low clamp mech from a friend to see if it will fit and if it will make any difference, not tried yet though
3) Getting the BB off might be hard, my torque wrench goes to 100Nm and that wasn’t enough to get it off, it’s only supposed to be on at 40-50Nm. I had to buy a breaker bar in the end.