Retarded looks and weird weight balance aside, I can't see that being faster than transitioning to a holstered sidearm on your belt. It adds so many problems to the mix it isn't even funny.
The way I was taught was if you needed a pistol transition because your rifle was down, you let the forend of the rifle to your left, while your right hand started doing exactly the same thing it would do to grab the sidearm, release the retention and being the draw stroke. If done/timed right, when your drawstroke from your right hand is to your chest when you are about to push the sights out on the target, your left hand has completed 'stowing' the rifle out of the way and meets the pistol/right hand to complete your grip and then push sights on target.
The benefit here is if you do this correctly, its using 90% of the exact same range of motions that you use when practicing a draw from the holster; nothing different, no new movements to remember, no confusion, nothing. You're basically using the same drawstroke with simply moving the primary out of the way first, thats it.
Whereas with this giant buttstock, its a totally different set of movements you'd have to remember and automatically do while under stress. Dumb. As. Fuck.