Contractor wants to "derate" my subpanel
Yeah, that's a thing.  The problem is you have potentially two energy sources feeding the panel: The 200A main breaker and the 60A Tesla PowerWall.  That's 260A on a breaker bus that is rated either 200A or 225A - I suspect the former, though the label will say for sure.  

Now, they give you a mulligan for 20% of the panel ampacity. So they'll let you slide up to 240A on that 200A busing.  But you need 260A so that is not enough. 

So, one option is to derate whatever breaker happens to supply this panel. Now you have   175A + 60A = 235A = you're inside 240A. 

Another way is to go talk to the dealer who sells those lines of panel.  They put a large variety of panels on a limited number of panel box sizes.  The goal is to find a panel with 225A-rated busing, where the bus assembly will bolt up to the same mounting holes that are in your panel.  

Now you'll have 225A x 120% = 270A, which covers your 260A.