Not really. Firms hire based on the cost of an employee. If suddenly the employer portion of the payroll tax disappeared, there would immediately be an incentiv

Not really. Firms hire based on the cost of an employee. If suddenly the employer portion of the payroll tax disappeared, there would immediately be an incentive to funnel those funds into the salary to make job offerings stand out against what the employee's other options would be. Sure, some firms would pocket it, but it beggars belief that most of them would, when they've already made a decision to open a hiring line based on the funds available and projected revenue.