The pedal travel shouldn't really be any different, certainly shouldn't be noticeable as the A4 MC is more than up to the job, all B7s use the same one including the RS4 so it's well up to the job.
Totally agree with Carl, they aren't the easiest things to bleed and can take several attempts to get a good pedal. You started with the inboard bank first?
The 17z pads are a much better fit and will avoid the lip that builds up on the 18z pads after a while, changing them over would be a good oportunity to try another bleed.
The only other potential issue are the actual calipers, not all 18zs are the same. The Q7 and Touareg ones are the ones you really want as they have the smallest combined piston volume, easily spotted as they generally in an unfinished condition. Avoid factory finished red ones as they come from the Cayenne turbo S and have the largest piston volume. The factory finished silver ones from the lower cayenne models are ok to use, I think their piston size is somewhere in between but still safe to use. Bizarrely the 17z although slightly smaller actually has the largest piston volume so best to avoid.