alt Hacker NewsAs I pointed out elsewhere, "X+Y... would all be salary going to you if not for the payroll tax" makes the assumption that companies are currently paying 100% of what they could possibly be willing to pay for that employee's labor. Given the profitability of California's companies, I suspect there's some surplus there. And a surplus suggests that the value of the labor is being driven moreso by what price will attract sufficient employees, which would only change due to 2nd or 3rd order effects by the elimination of payroll taxes (via competitors willing to pay more for a finite pool of top laborers popping up).
I believe it is a reasonable hypothesis that if payroll taxes were removed, 2nd order effect would be that employers have more money to offer for all positions, and in a market driven job market, prices would increase and thus salaries would converge to X+Y, yet they would be worth the same as X today.
Yes, likely not exactly the same (a bit more kept by employers in overcrowded job markets, a bit less in others), but it would essentially support the interpretation that most of that is really a tax that goes out of employee "budget", or their total comp.