I have some M8 GMK Spherical samples, and the keycaps "bottom out" on the top housing of regular Cherry MX switches due to the shorter key-stem of the M8 keycap underneath.
I must say though, the M8 sphericals feel way nicer than SP's DSA since all of them are pretty much shaped like SP's homing "scoops" & are really smooth.
Not sure about the thickness of vintage Cherry M8 caps (as seen in Ivan's pic), but the M8 DS sample I received is ~1.7mm thick, with the top portion (second shot)~0.82mm thick. Whereas SP's ABS/PBT DSA caps are ~1.27mm thick.
Some pics