It depends on how sensitive the new cab is. You could probably go a little louder without distorting your sound but (as Lozz says, depends on your choice and the rating of the cab). You might also find another cab that gives you a better tone, but the difference between 500 and 800 watts will not be as much as you expect. Won't be a 50% increase in volume. I used to own an Epifani 410 rated at 1000w and I still needed to add a 210 in one of my bands to get enough volume for the mad guitarist.
You can add a 210 to your 410 which will give you more volume and tone. Just remember to keep to within the spec of the 210, ie don't over drive the smaller cab. That's still 2 cabs and is probably the cheaper solution to your problem.
But if pure volume from a single cab is your requirement. . . . then the cab I was referring to in my previous post was the Barefaced Four10. That is the only 410 that I know that will get close to an 810 is terms of clean volume. I have the Two10 and they sound pretty good.