One issue I've heard raised is that some PIs like to use salary differentials to reward the "good postdocs" with bonus pay.
Given the behaviorist education that lurks in my background, I am theoretically* in support of this notion.
The new salary rules may minimize such flexibility in the future.
Are you aware of labs in which merit of postdocs as interpreted by the PI leads to salary differentials?
Is this a legitimate complaint about the overtime rules?
Will PIs use the permission to work overtime (and be paid for it) as a workaround for merit pay?
*Given my distaste for workplace bias and desire to be a fair manager, I have never used merit to decide postdoc pay. I stick to NRSA schedules and to institutional adjustments as appropriate.