Answer
 

No, you can't lower the differential mount.

.

couldn't you space it out with longer studs and spacers on the stud between the diff and the rear subframe?

Im imagining you would also have to shim the carrier bearing mount out as well, but if you did both it seems to me possible to drop the diff some, after all there is that CV joint that seems to have no purpose on these cars.

Seems theoretically possible, but for a diminishing unlikely advantage.

OK
 

Your theory is reasonable.
However I have tried it many times (customer demand), and the studs broke and/or sub-frame cracked.

I can only assume the positioning is critical for stress tolerance.

**************************

IMO; The only reason his CV boots failed was age = they would have failed shortly, regardless of any other factor.

.