Hey Pierre,
Not sure if the 2 are related but presuming they are...
I've just been testing an ESI UDJ6 soundcard and the image I backed up before updating the firmware would only run the card with the usb and ethernet slowdown options. Post rpi-update got 6 channels of audio without either slowdown required though I think it does run better (less system resources) with both options enabled.
My trusty ol' Griffin just 'works' with both installs so didn't notice any difference but I haven't yet checked whether usb slowdown is still required for duplex with that soundcard as of yet.
This is on a rev1 256meg board btw.