I have fitted a Corbin seat and it does not interfere with Kawasaki top box or Shad 48 am with coldcomfort might be the backrest option they are referring to, I have completed 20,000 kms with no backrest so not an issue for me.

Ken Pyke
GTR-AUS # 118
COG # 8922
FarRider # 82
IBA # 62198
Ulyssses # 35827 
1995 Kawasaki 1000GTR Kawasaki Racing Green "Kermit"
1986 ZN1300 Voyager "TBA"
2008 Kawasaki 1400GTR Black "Le Noir"

Lives Burnside Heights Vic