The largest difference between specially commissioned drivers and generic drivers is the drive impedance, and certain damping characteristics. It merely makes a designer's job easier. It takes a lot of money to specially commission a driver that's very obviously superior to the generic variant, the most prominent example of which is the driver unit in the K3003. But few drivers are like that, not even the ones from Westone or JH. UE seems to take advantage of their volume sales and large financial backing as well, but even the designs they commission are not too far away from stock.
CIEM design has much more to do with tube length, damper choice/positioning, RC choice. These things greatly influence the final FR, impulse response, peak amplitudes/frequencies, phase/timing issues, etc.