Quick update: considering if (IF) the PAC AOEM-MAZ2 works with the CX5 2017-2018, which I believe it works, anyone willing to bypass the Boss Amp, can do it. Isn't the easily task but I found out few things (at least for the Sport Nav version of the UK):
As mentioned before, the speakers are:
Central Dash - 3.5";
A Pillar - 1" Tweeter (mount is a bit bigger, so I think that a 35mm would fit in the original slot;
Front doors - 6.5"
Rear door - 5-5.25"
D Pillar + 3.5"
Sub Box - 5"?
About bypassing the amp: Once installed the PAC AOEM-MAZ2, connecting it to the amp, the feed from the amp to the:
* Front Doors - First the connector from the car's boby, not the door yet. There's 4 small clips holding the connector under the rubber grommet. You'll have to disconnect the grommet from the connector, with a thin object (something that won't scratch the paintwork, press against each pin. Once the plug is disconnected, using a guide cable or a cable tae, you'll find a gap next to one of the connector's border that will allow you to run a cable. Note that the cable has to run from inside the connector as if going outside the rubber grommet won't seal properly the gap and water may ingress. After that, you'll remove the door trim. There's plenty of detailed guides how to do in this forum. Once removed, there's a square-shaped grommet were the cables run from the door to the car. Disconnect it, tem a grey connector is visible. There's 4 clips holding the cables in place closing the connector. Disconnect them, them you'll be able to run the new cables using the same gap as the other cables.
* Rear Doors - You'll have to remove the lower trim which cover the seat belt mechanism. (just the lower part of the B Pillar). Once removed, you'll have to disconnect the rubber grommet to expose the connector. There's only 2 pins holding it in place. Here the work is harder. You may have to disconnect the stuff holding the cables to the pin, as it allows you to "maneuver" the connector inside the vehicle, which you'll need to install the cables. You'll note that there's few empty connections in that connector, but no gaps to simply run the cable. Some more advanced DIYer would create those connections in both ends of the connector and sorted. I'll go for the reckless mode. Using a very thin screw driver, you may be able to poke through those two connections without damaging the connections next to it. Note that the screw driver and the cables used later to feed the speakers have to be thin. The connection to the door is similar to the front doors, the only difference is that instead the cables coming straight from the door to the vehicle, there's a terminal which would connect to the B Pillar's connector. Note that there's about 3 empty connections there, same as the Pillar's connector. Same method. Don't try to run the cables using the upper gap were the connector have a clip as it may break the clip or don't allow it to connect properly.
For the tweeters, central dash speaker and D pillar speakers the connection is a walk in the park.
I've ordered the bits and will update with pictures, as would make more sense the method I'm describing.
The idea is to avoid cutting cables. The only mod that won't be reversible is the stock door speakers as they'll be used as bracket for the new speakers.