Funny you should say that . Mark ( Grandtrousers ) has a really nice custom made underseat exhaust system on his GSA and is currently looking at luggage system with 2 full size panniers to take advantage of his 'pipe'.
He had his pipe made by some outfit in the NW who's name escapes me.
I finally got my 'Civils' fitted last week (wife demanded I go to London with her and the kids!) and refitted the Adv rack. All looks great but the lids are a faff to get on and off becasue of the rack's love handles.
Since this was taken I now have a slightly smaller numberplate which sits nicely under the tail light and have relocated the indicators on a custom made bracket (they sit "nearer" the number plate)- oh and I have moved house!