Hi Rolf, here my ideas why some customers made changes to their M20.
* Carbon rigging is a bit sensitive and you can easily cut a stay if you get a fast running halyard wrapped around it.
* Most T sailors has switched to a rear block system to keep the tramp clean for spi work, and it also a matter of "fashion", if the top guys use it the rest will follow. The cascading system has a limited "stroke", if you choose 4 times purchase between boom and beam and pull with a 2 times purchase on along the boom you have a "stroke" of boom length / 4. On the M20 casading system they use a 3 times purchase along the boom, a total of 12 times purchase. The limited stroke can sometimes get you into trouble in extream weather.

I don't think the hulls has been reinforced but the rudders and the centerbords are reinforced. The rough conditions in Holland required stronger foils! (Mitch Booth used M20 rudders and centerboards for his experimental Hobie design).
I'm not sure about the mast, it started with a single spreader with double sets of diamond wires to stabilize the mast section but was later switched to single set of wires and high modulus carbon fibre, and maybe more fibre also.

/håkan