The 04 cluster is different from the 05/06 cluster.(Font and redline RPM). Not sure about programming or plugs(would assume its the same).
A4 and M6 are different. A4 shows the gears the trans in in and the M6 doesnt. I would assume they are interchargeable but you will have the annoyance of seeing PRND123 or so on it perminently.
The miles are saved in the cluster. If you get a replacement have it installed by a shop so they can record the miles on the old cluster and the new cluster at the same time or you could be in for some headaches down the road. Also, you might throw a tamper code somewhere but it won't show up on the DIC or anything but I'm sure a GM dealer can see it.
This is all from first hand experience because my RPM needle broke and I swapped clusters before I realized I needed to just replace the needle assembly to do it properly.
2003 Corvette Z06 - MGW shifter, Corsa X pipe, Corsa Xtreme exhaust, Texas Speed long tubes w/o cats, PowerStop rotors/pads, Conti DWS tires, C6Z51 F&R sways, C5R headlights, tuned
04 GTO - sold