That's a lot of questions. I will do my best to answer them but note I am not trying to sell a language here. I'm just a student and wanted to show an interesting project I'd been playing with.

The performance implications are much like vtable lookup in C++. Generic functions are essentially overloaded and their function pointer looked up at runtime. The main tradeoff over C++ is that rich types don't play well when allocated on the stack as destructors cannot be automatically called on frame exit.

The added semantics are not optional, but you can still program in standard C while avoiding the defined keywords and functions and the interaction should not be troublesome at all. There is no price for linking the library.

It is not type-safe and employs runtime type checking where inserted by the user - but also this allows for duck typing and more generic code so perhaps it is not a bad thing all round.

I hope that helps. Any more questions feel free to drop me an e-mail.