In other words, you guys believe that my problem with cutting the mix should be more the speaker than the amount of speakers.
I was also hoping that the slant would help with being able to hear myself better on stage without being too loud.
Adding more speakers will not help clarity at all.
You may not need to change anything at all. If your cabinet is sitting on the ground, then honestly you have absolutely no idea what it sounds like out front. While you think it isn't cutting through, most likely it's cutting right through the eardrums of the people in front. Place it off the ground (at least on a chair if not a stand) as far behind you as you can - this will allow you to hear everything that's coming out of the speaker.
You may not need to do anything. That amp though a Mesa 12 ought to be very clean on a clean setting and the break up nicely when over driven with only 15 watts. Try varying the placement first and work with the EQ. You might save a bunch of money.
If that's still not what you want, then consider changing speakers - but, moving away from that driver to a lower wattage one will only serve to add more break up and less clarity.