1. Most of the underlying "Dark" is similar between the Dark Basic and Dark GDK implementations from what I have seen thus far. The GDK is basically the C++ implementation of Dark Basic, close cousins. Code written for Dark Basic will not work with Dark GDK.
2. I haven't messed with shaders much. From what I know, the function calls and setup are just the same,though.
3. Nope. Basically, the portability to the Xbox 360 is thanks to the
XNA framework. Visual Studio itself is just the compiler/IDE for the development and like any C++ compiler, it "can" make applications for any environment but library usage and compilation settings are wildly different. It would be on par with trying to jam a Nintendo Cartridge in a Genesis. Both have game code on them, but only one is made in a way that the system can handle.
Using DGDK since 01/01/2008
Blah - http://www.john.goewert.org