I started to play one game the other night with my cue extention (8 in) threaded onto the butt of the cue. I had used it on a couple of shots while playing scotch doubles (my partner must have enjoyed leaving me those hard to reach shots). Anyway, I decided to just leave the extension on for the next game.
It is a very light plastic extension, around 1oz or lighter, and it probably only made a small difference in the balance point of the cue. However, the difference in how it felt when stroking the cue was not small. It was noticeable enough that it affected my stroke, my timing. It didn't feel right, an uncomfortable distraction, and I found myself trying to adjust my grip hand position and bridge length to fix it. Still, nothing felt right. After a few awkward feeling shots, I took the extention off.
I'd likely have to play a few hundred hours using the extension in order to develop the muscle memory that would make it feel normal.