That is probably not going to happen. No matter the wattage specs, a single 1x15 is unlikely to be able to handle the volume of a metal band that is loud enough for you to be worried about competing. As has already been said, a 1x15 will not be as loud as a decent 4x10, so unless your 4x10 is *easily* able to handle your power needs by itself, a 1x15 by itself probably won't cut it.
This is a good point. The same could be said for my 210 recommendations.
OP, how easily is the 410 cutting it on its own now? For a second cab, are you looking for something more portable, or just more volume sometimes (practice vs. playing a big room)? Can you fit two 410s in your car?
Just to throw out more options, if you *did* end up with an 8 Ohm 210 or a 15 and had a second head (you mentioned the D800+ being your "main" head), you could slave that to the D800 and control the volume of the cabs independently. Not sure I'd want to do this long term, but you could do that until it was practical to switch your 410 to a 4 Ohm cab. This would also let you evaluate how you like the 410/210 (or whatever) setup before your drop more money on it. Again, an identical 410 would avoid this, if it meets your needs otherwise.
There are a variety of modern cabs that'll get you louder in a smaller space -- some of them have been mentioned. I've liked the Audiokinesis and MAS cabs I've played. They are worth looking into. If you have the budget and dig what they do, they can be a worthwhile investment.
Regardless, if you get two identical cabs, I'd go with 8 Ohms each (unless they're stout indeed). You'll get the same power from the D800+ at 4 Ohms or 2 Ohms. If you get one moderately big "do it all" cab, you'll most likely want 4 Ohms. Three cabs?
While I like the idea of a bigger 4 Ohm and a smaller 8 Ohm cab that can be used together, if the small cab can't hang on its own, it would be relegated to home practice and adding some volume to the big cab on louder gigs -- that's not necessarily bad as long as you're aware of your needs and go into it with eyes open.
Ultimately, you'll have to play something to know if it's for you, but if you do your homework you'll better your chances.
Good luck!