Elizabeth Warren's Medicare Plan reviewed

Here's a good critique of Warren's plan by Jordan Weissman in Slate.  He points out that the "employer medicare contribution" is in effect an head tax on employment, and would cost the employer the same for every employee (since it's calculated as the average amount the employer currently pays for his employees' plans). Starbucks would therefore pay the same amount for the CEO's plan as for the barista's. And if Starbucks adjusts wages to increase or decrease each employee's wage  by the difference between what the employer is currently paying for the employee's plan and the uniform "medicare contribution" amount, the CEO would be a winner and barista would be a loser, assuming Starbucks currently pays more for the CEO's plan than for the barista's.  But this raises the question: to what degree does the amounts employers currently pay vary according to the wage of the employee? If there's very little variation, then switching to the Warren's plan wouldn't change much.