I'm not an electronics person, but I have been playing for a very long time and I know what I hear. The difference between one 410 and 2 410's is pretty much always a very huge difference...my favorite rigs have always sounded best at 2 ohms...I understand the heat and shortened lifespan, trust me...but I really don't care...it sounds better to me.
And for the most part, all this mismatched 'phase' talk has some merit, but it's a bit overstated...I don't like to run 10's with 15's...the 10's move quicker than the 15's, which is where all the 'phase' talk comes in...the time alignment bugs me a bit, so I just use a bunch of 10's...for the past few years I either ran eight or sixteen 10's, depending on the venue size...overkill? Oh yeah! Of course it is, but it sounds really great, even at lower volume, just moving all that air , so to me it's well worth it.
Also, switching from 8 to 4 ohms should be a very big difference, regardless of what anyone says here...sorry, fellas! Personally, if I were in the OP's situation, I'd ditch the existing cab and get a 4ohm 410...then if you still want more juice, get a matching 4 ohm cab...stacking two 410's has a bunch of advantages...getting the mids and highs up closer to ear level...it just sounds better.
I do envy the players who can be happy with a single 10 or 12, but I can't enjoy myself unless some air gets moved around, but that's me