The Leica SF 40 has a guide number (GN) 40 when using a 105 mm lens. For using with a 35mm lens, the GN is 35. It is true that the size of SF 40 is relatively small, at 84mm high. A normal flashgun will be around 140 mm.
Many forum posts mentioned that the Leica SF 40 is not an original Leica designed flashgun. Like some of their flashguns made by Metz, the Leica SF 40 is probably a rebranded Nissin i40. Leica does not mention any information regarding this, but if you replace the brand logo with a Leica one, you won’t tell the difference between the two. Nissin is famous for and has a long history of making flashguns compatible with Canon, Nikon, Sony, Fujifilm and Micro Fourth Third system camera.
Could we use the Nissin i40 on the Leica M? I have no idea. But I won’t try this out as a non-compatible flash may may cause some significant damage to the camera. You won’t be surprised to find the Nissin flashgun may cost only 50 percent of the price of that of the Leica SF 40. Don’t forget everything branded with a red dot costs a bit higher.
How about other features? Stay tuned.