More cone area almost always equals more output.
Will two tens + two 15's be louder than just 2 15's? Yes.
Are there issues with things like phasing and cancellation? Yes, but these are issues with any multiple subwoofer system, even if they are the same size, brand, age, etc.
In a perfect world, would I mix sub sizes? No, but if you have them, and want to use them, it will 'work'. By work, I mean put out bass.
Will it 'even out' response, as you suggest? Possibly, but that would either be through extensive design work, or pure luck. Not worth the effort, for the most part.
I won't enter the other arguments on this thread, other than to say that Fs is a key factor in box design, and will have an effect on how the system sounds, and what ranges it can reproduce. All other things being equal, a driver with lower Fs will play lower, louder, than a speaker with high Fs.
This is precisely the issue. Do not run subs of different sizes unless you have them operating in different frequency ranges. Different wiring methods and different amp configurations are not going to help you unless you have isolated frequency ranges.