I imagine it doesn't need to be a lower level of sharingan than itachis. This reminds me of the whole genious, umm... hard work thing. The sharingan wouldn't have even been possible without perhaps an eye somoene from Itachi's clan. Kakashi has the eye, but isn't necessarily what you'd call a true genious with it(not being an eye he was born with, or born with a body wired for it) It seems this sharingan still takes a toll on someone born with the sharingan, but perhaps less of a toll than it is on someone who has acquired it kakashi's way(depending on its use, kakashi could very well lose sight in this eye-is it accepted that this sharingan affects the vision?) If it is particularly draining on kakashi now, imagine when he first developed it, and consider the future.