I don't think he want to loop 257 bytes in 60us. He wants the copy to take 60ns/byte, i.e. no slower than 15.42us for 257 bytes :)

Something to think about when you do a loop with pointers is that a lot of processors (or compilers) will like the following better:

char buf[257];
char *src = buf;
char *end = buf + 257;
char *dst = &FIO_PIN[3];
while (src < end) *dst = *src++;

The compiler should notice that "end" is a constant and that it can drop the it (or at least reduce number of modified variables) and instead compare your pointer with a constant to decide when to break. Besides helping the code, you may also reduce the number of used registers.