Yeah yeah my good man. Thats the mandatory answer from socalled "innocent" people. <innocent whistling>
The reason for the locked values is homologation in real life. You can just change the preload of the diff there, too (for example GT Masters and Blancpain), the balance of performance shouldn't interfere with diff settings, it's more of a weight and restrictor thing IRL.
GTE and GT3 in rF2 are not following a specific series though, but you are right, but also note that even the preload setting is locked in the Porsche 911 GT3-R and the BMW M6 GT3 (in rF2 i mean of course).
They aren't a specific series, but have a series on their own in rF2 kind off. The GT3 Homologation isn't tied to a specific series, though.^^ Yeah, locked preload is a strange thing...
IMSA regs 2017 say that the manufacturer has to seal the diff along with the transmission either by the PMNA or PMNA partner
Does the source specify just for locking the behaviour under coast and power regarding homologation (usual regulation in many GT3 series) or also explicit forbidden preload adjustment too (adjustable part in many GT3 series)?
That's all that was written, so I'm assuming the manufacturer has to find the best compromise and leave it there. Like how F1 locks the ratios of the gearboxes.