So far, of the cars created BEFORE the new sound engine became the standard, only the Oreca 07, the Cadillac DPi, and the GTE cars have had their sounds fully updated. Cars released since the sound updates, such as the BTCC cars, all have had the full sound package when the cars were released.
That could be a rather extensive list. We could list which cars still have the original cockpits. Which cars are NOT on the newest tires. Which cars have not received the full sound updates. The problem with updating older cars, even DLC, is that it doesn't justify the cost. I mean there is not going to be a large uptick in sales of the GT3 pack simply because the cockpits got the full shader treatment as most users already have the pack. It WOULD generate good will, but that does not bring in $$.
Now with LFM i think the gt3 update is a must. rF2 series needs more races there and what better than GT3s? I think 397 is taking its time to fully update them , because of competition with ACC.