I see little value in that. A person bringing both the 24 and 40 and possibly something else might be better served by the 17-55. Either that or pick up the M with the 22 f/2, which is still less inconspicuous than an APS-C body.This is why: http://camerasize.com/compact/#448.303,448.345,ha,t
For me, I do adventure sports, which either involves long treks, or being put into a small case in the back of my boat. That weight makes a very big difference over miles, and it effects how the camera handles. The EOS M can't AF, which makes it of limited use for multi-frame bursts. It's the same issue with basically all the mirrorless cameras; I need portability AND the ability to shoot action.
Moreover, if the new lens ends up being about the size of the 22 STM, I can own it and the 40mm pancake and still have $400+ and 1lb of weight saved. If it ends up being f/2 instead of f/2.8, then its even better off than the 17-55 (IS is of no use to me in action shooting)
That's a great size comparison except if you need to bring other lenses for other focal lengths anyway...
I hope you're right about the new lens being the same size as aperture as the 22 f/2, but I'm guessing it'll closer to 28 f/2.8 in size rather than the 22 f/2.