Mega Man 6 works without any problems (tested the game myself), so I guess that Mega Man 2 should have no problems too. In regards of Mario 1 + 3 there are no reported issues. I once tested the 2-in-1 Mario + Duck Hunt US cart and Super Mario Bros. run flawlessly. Castlevania shows an issue within the status bar (scrolling effect). The game itself is fully playable. For the other US games that you listed we have no reports yet unfortunately.
May I ask you why you won't just go for the PAL versions of those games? In many cases PAL versions have tweaked music speed or even gameplay speed (the most prominent example is Super Mario Bros.). So in those cases the US carts run even slower on a PAL console than the PAL versions (I have no elaborated list of tweaked PAL versions at hand, that would be a cool project for some other day
). Another option would be to invest in a US console.
Obviously our little database project is more interesting for those titles which happen to just not exist as a PAL version (i.e. Mega Man 6, Startropics 2, etc.).