To my knowledge Tesla is marketing panels in units of 4.25 kW, so their inverter sizes are also going to increase accordingly. I.e. the standard DC/AC ratio is fixed and their price per watt covers both panels and inverters. I seriously doubt that if you have a 8.5 kW system being designed with them, and you say "oh, I can fit in two more panels, what would that cost?" that they will give you same price adder for 2 panels only vs 2 panels plus an inverter increment.
If you like, I'll amend my statement "if your not clipping, your system cost too much" with the footnote that maybe you got lucky and the system was mispriced and some of that cost was eaten by another party.
Also, depending on what you care about, certainly at DC/AC ratios just about 1.0, the net environmental impact of increasing the inverter size will be negative. As it will only slightly increase energy harvest, and that increase will not offset the environmental impact of building that extra inverter capacity.
Cheers, Wayne