Where the FAA has gotten concerned in the past is where they are
specifically sized (TCM, for example, makes three sizes of injectors
for the same engines). What a lot of people were doing was ordering
some of the "other sizes" and swapping them into the mix, in an attempt
to even out the fuel flow. Or worse (in the FAA's mind), simply using
an abrasive wire to open up the injectors themselves.

Right. And to get consistent fuel-air distribution on the big
Continentals, you pretty much had to do that (or go to GAMI's). But I
have small-bore Lycomings. They have pretty good fuel-air distribution
to begin with, so the manufacturing tolerances on the injectors are
enough to get the job done. That's why GAMI's are pretty much a waste
of money on small-bore Lycomings. You can get the same effect by
swapping the stock injectors around. They're supposed to be identical,
but the manufacturing tolerances are such that they are not. (My
favorite tall tale is the one about Eli Whitney and the interchangeable
parts.) They make a lot of sense for big-bore Continentals.

Michael