The research i have seen, has suggested that whilst it can slow down the original speach for a child, the long term advantages in so far as the child can communicate far ealier have only ever been advantageous.
I am not a mum who looks short term, I am interested in long term goals for my child, and having a HAPPY content child right now. Our daughter has some chronic food intollerances, which really make her uncomfy and put her in a lot of pain. From 9 months she was able to tell me exactly where the pain was. this enabled a far swifter pain relief, and us to eradicate "toxic" substances from her diet far faster than if we had remained waiting for her to "talk"
This for me is one of the main advantages of sign. She still uses it for words she finds hard to say. We encourage her to say the word, but at least at the time of her excitement, she is able to communicate what she wants, and we later help her say the word... elephant and tomato come to mind... :d
Iwill try and find the links i read, but it was over 24 months ago now.