If my memories are correct, Steve isn't the only one; didn't G-virus-mutated Curtis from Degeneration also remember... something when he found that photo and looked at it with his giant eye? And while Birkin had certainly gone nuts, at least he still remembered who his relatives were.
Then we also have lots of characters who (at least partially) retained their sanity and intelligence after mutating: Wesker, Alexia, Irving, Piers, Carla, Simmons, Morpheus, Manuela and her mother come to my mind, but I never finished RE4, so maybe there's more... Doesn't Krauser qualify as well?
With that said, I think it always depends on the person, the virus, the way the person reacts to the virus, and perhaps even the circumstances of the infection (it does seem so with the C-virus), so there's no definite answer to this question.