I think someone mentioned this before but there are several wear items that should be replaced or at least I did when doing mine. E.g. Strut, strut boot, strut mount, coil spring insulator. I believe that's it but I would need to check my receipts later to be sure since its been a while. I reused my old springs when I did mine. The KYBs (GR2) will not accept the original GM boot so that's why I ordered new ones. Actually my old boots were fine and a are far superior to the ones KYB sells. Not sure if they have improved.
btw, the rubber accordion thing is the boot.