I don't personally feel it is too light as modern F1 cars look quite light on the steering, I felt as I do in most games if I go heavy that I am missing out on some fidelity. Obviously there is a lot of personal preference with any settings, just turn up the wheel weight a little until you like it. Also there is probably some variation in wheel bases to take into account.
I have not felt the loss of strength that you mention in longer races.