I am also flyin an 8 with the same issue. In my case, the vibration is not too bad with my feet on the rudder, but as soon as i put my feet on the floor the vibration is very bad. From reading other posts, I've heard that its not so much the exhaust as the general vibration from the engine/prop. Could be wrong, but I'm in the same boat as you lookin for less vibration from the floors.
Sling Tsi- (under construction)