What will or won't happen if I use a 6" pad with the 5" counterweight?

 

Not much performance-wise, but it may become less comfortable to use at higher speeds (5+) because the vibrations are a bit worse. Over the life of the tool I don't know if it would shorten the life of the bearings though. My suggested "best" combo would be a 5" backing plate with the 6" weight. I've never tried the big 8" foam pads but 6" ones seem most popular.

 

It defomately shortens the life of the tool and also causes excessive vibration. Theres plenty of kits out there that sell the counter weight with pads and other accessories.