Are aftermarket intercoolers legal for stock?

[Buick6]

I wondered about this before as it applies to older turbocharged cars like the Buick GN, Mustang SVO and turbo Thunderbirds. On those cars, how many tech officials know what the stock intercooler looks like? Stock intercooler cores could be changed to a more efficient bar and plate design from the old tube and fin and hose openings enlarged to gain a huge improvement in flow. Legal? Of course not. But virtually unnoticeable unless you know what they look like.