You don't need to use Intel's FSP if Intel will tell you how to do what the FSP does. FSP doesn't do anything magical, it just happens to setup a whole lot of early boot things for you like SDRAM configuration, microcode loading (and FSP knows what instructions its not allowed to use before microcode gets loaded), and starting the ME.
For Intel Bay Trail parts (which Intel publicly says you have to use FSP for), I was once told by a BIOS/firmware vendor that they didn't use Intel's FSP because it was too slow and their customers wanted very fast boot times. No idea if they were pulling my leg or not, but Intel's FSP on Bay Trail wasn't exactly super speedy.
For Intel Bay Trail parts (which Intel publicly says you have to use FSP for), I was once told by a BIOS/firmware vendor that they didn't use Intel's FSP because it was too slow and their customers wanted very fast boot times. No idea if they were pulling my leg or not, but Intel's FSP on Bay Trail wasn't exactly super speedy.