Janis' screeching, as you label it, is part of her trademark style, a sort of vamped up hyper-emotional blues style which tears the vocal cords apart in its urgency of expression. Bobbie McGee is probably not the best sample to use to appreciate the power of the style. It is more melodic than a traditional blues form, and more of a ballad in some ways. Find some core blues numbers she has cut and it will seem, perhaps, more appropriate. She was a very whole-heart gutsy singer, took a lot of risks musically and personally, died young.