My guess would be zero more than now. None of the startups is limited by getting employees and I'm almost certain no funding would be gained by allowing at will employment as it is a non-issue anyways (as it is already possible to do so by just hiring contractors but nobody does this).

edit: Consider that most people would request considerable more than they earn here now if at-will employment would be the case, offsetting any "gains" you might consider. In Europe wages are considerably lower than the US.