While not a CX-5, I have a '17 3 with Bose and I too was frustrated with the lack of detail in the higher end of the spectrum. I tried to tell myself I was wrong for a few weeks, but decided I had to get a real tweeter in the dash.
The good - as you have discovered 3.5" bolt in. I had to do some minor plastic trimming of the bracket that holds the speaker wire inside the dash, but that's about it.
I haven't used the Infinity's, but am thinking of trying them. As my issue was mainly with trebble I went with the JL Audio C2-350. I went with these because I had C3-650's in my gen 2 3, amplified, and was very happy with them. The JL 3.5" were the only ones I could find locally with a silk dome tweeter which I know I prefer. I thought this would be especially important with it being located so close to the glass. I didn't want a low end tweeter to be too harsh considering the location.
Here are the pluses and minuses of the way I went:
Pluses:
- Frequency response is greatly improved
- Treble is much more present and much clearer. I listen to a wide array of music and cymbals present themselves well now musically
Minuses:
- These speakers are much less effecient than OEM. I knew this going in and I thought long and hard about going with this speaker at first but I couldn't get over the fact that it had the tweeter I wanted.
- The speakers work fine, but due to the Bose tuning and the rest of the system the balance does get thrown off and you will have to tweak it to get it as close to back in balance as it was before the swap.
- There's no way to tune / balance the sound of the dash speakers with relation to the center speaker.
Neither Pluses or Minuses:
- As mentioned, tuning / fiddling is required. Before the swap I was
--- +2 on treble
--- anywhere +1 to +2 on bass to try and keep it present (much more than that and it got muddy quick especially at moderate or higher volume levels)
--- 0 on fade. I quite liked the OEM soundstaging
--- I bounced between using centerpoint and not depending on what I was listening to. Some music I liked it, some I didn't. Overall it was on more often than it wasn't.
--After the swap I was:
--- 0 to +1 on treble
--- Anywhere -2 to 0 on bass
--- +1 fade to the front
--- I now only use centerpoint. Without centerpoint it feels like the center is still on and heavily biased.
- So why did bass get adjusted? With the lower efficiency the volume know goes up a few notches higher to listen at the same actual sound output as before the swap. With the volume knob going higher to hear the new dash speakers at an ideal level the rest of the speakers were too loud, so fade went to the front to quiet the rear a bit and bass went down to balance the door woofers with the dash again.
- Overall this has made me happier with bass response out of the door woofers as well as it's more present with the quieter dash and the bass is cleaner in the -2 to 0 range at higher levels.
With that said my normal volume level is now in the 40-45 range on the dial when hwy driving on my own to enjoy the music. Where I think it was 35-40 before.
What makes me curious about trying a more efficient speaker is if it blends better with the OEM tuning with the center speaker as it is more present with the decreased effeciency of the side speakers. With my setup there are times when it sounds like the corner speakers are quieter than they could / should be. Lower volume levels are also tougher. I almost always have to drop the bass down when at lower volume levels or it's overpowering.
I also wonder about putting the OEM speakers back in and adding an independent tweeter, but not sure if the Bose amp can take it. I'm not sure how the Mazda 6 is wired up with it's twiddler / tweeter combo and if the 6 amp is any different than what we have.
Take that for what it's worth. I hope it helps. I'm used to fully aftermarket systems picked and tuned to my liking but I'm not going there with this car. I simply wanted to improve treble.
In retrospect I wish I had chased down some 351's as they're clearly more efficient, but as you mentioned they're discontinued and I can't find them anywhere here in Canada.