WOOHOO! VTAK YO! Why does everyone always say to keep the VTec head? The A6 flows VERY nicely. You will however need a non-vtec ECU or else you'll be throwing a code for VTec. Another problem is that the A6 is OBD0 and Y8 is OBDII... so if your vehicle is meant for OBDII, then you'll be throwing codes for any and all sensors on OBDII that aren't on OBD0.
A6 head isn't so good if you go N/A, because this head has 1mm smaller. Combustion chambers are also bigger, who reduce compression ratio. But I heard that valvetrain is stronger and spring stiffer on A6. I think if you go turbo it could be a good idea. REMEMBER: Valve location into A6 head is not the same as the Y8, check your pistons valve reliefs with clay to be sure.