I have fitted the Voight 25mm bar risers to my R80 G/S ... these fitted into the existing lower 'cups', albeit not a perfect 'line-up' and you will need longer bolts than those provided in the kit, I also found that the cabling did not require any extending, there was plenty of slack to accommodate this change.
I’ve got some as new switch gear that have had the main loom connectors removed as part of a bespoke loom build might be of use if you are going to the trouble of extending