I use it with SLB300.
It is a must if you want to get close to the feel of the "real thing". It deepens the instrument's shoulder that leans into your body, hence creating the illusion of having a thicker body. The neck's angle and distance change accordingly, and I would not want to play without it anymore.
It is beautifully made, comes with two screws and attaches in less than a minute. There are also two thin felt stickers that come between the instrument's frame and this extension, so that the two frames never grind against each other and (possibly) scratch the finish.
Why is this not a part of the original frame is beyond me. If we pay almost 4k€ for an instrument, this should already be in there. The price is steep, but a custom-made one would end up costing just as much, and would probably not be such a good fit.
The frame + extension still fit into the bag, although it now creates a bit of imbalance when folded. Also, this is now the widest thing in the bag.
Summary: absolutely necessary, no brainer.
