I've owned and gigged both the WA/Scout 12 and 15 cabs, and I agree that the 15 just fit the amp better. Handled more of the WA's 300w before distorting/farting, delivered more SPL .. etc etc ...I’ve had a Scout 12 with both 4 ohm and 8ohm speakers and a 4 ohm Scout 15 (which I still have) and find the Scout 15 to put out MUCH more clean volume than the Scout 12. Oddly I don’t actually find the Scout 15 to be significantly “tubbier” sounding on the bottom either. It has a cleaner low-end that actually sounds less congested and less farty at loud volumes IMO. The Scout 12 at 8 ohms actually sounded better to me than the 4 ohm and had pretty much equal max output as well...
The reason I only have a WA 12 left in my gear collection is the portability factor, and I only use it for very small/quiet gigs or rehearsals.
When I was using the WA cabs exclusively, my favorite combo was the WA12 on top of a Scout 15. Very nice combination even though it was a 2 cab situation.
It's interesting to me that Mesa somewhat promotes the 12+15 combination with its newer Subway cabs too.