So.. I'm going to try replying to this anyway.
Regarding DxWnd:
DxWnd is just so I can test fullscreen without having to actually run Cave Story in fullscreen. It makes things a lot easier.
Using DxWnd does seem to affect the FPS slightly, but altering Cave Story's FPS code wouldn't magically improve things.
Fortunately, this isn't really an issue, because as I said, I'm just using DxWnd for testing fullscreen...
Now the following is regarding my actual progress on hacking Cave Story to run widescreen:
Some things appear very oddly in my experiments with widescreen, but I think I've figured out the causes.
I can get widescreen more-or-less working in a window right now, but various things like NPCs, I haven't addressed.
And some final notes:
There are a bunch of projects I'm working on at the moment, and the widescreen hack isn't exactly a high priority.
I'm guessing Noxid would probably be perfectly capable of writing a widescreen hack, so I'm kinda stalling to see if he does