My DS could do around 70-odd signs by the time he was 15 months, but he didn't talk much till 20 months. He did acquire a small amount of language in the interim. He used a lot of signs because he couldn't talk yet, I don't think it delayed his speech. By 2 he was talking in 5-6 word sentences.
My DD seemed to pick up the signing quicker, but hasn't gotten as far with it. I think it's because she has started acquiring words much earlier than DS did, so doesn't need it as much. Plus she prefers emphasis over precision in communication.

