The M20 is not really a class boat, there are many different variations out there and I would think that there are very few that are the same specification.

Most of the development has seen Jibs added in order to rectify the reaching speed issues the uni rigged boat has. But other modifications included, different beam widths (now around 3.55m / 11ft 8 inches wide for the top boats) and now curved boards as well.

Take Xanders boat for example: it is now 3.55m wide with high modulus beams (no dolphin striker...) it has the same boards fitted as the Nacra F20 and a big jib.

Herbie's boat has the same beams, marstrom asymmetric section curved boards and a smaller jib than Xanders boat.

Will Sunnucks has 3.60m beams with a dolphin striker and straight boards, big jib etc.

These boats are all weighing between 138 and 150kg. (305 - 330lb) The F20 is about 165kg or 365lb

The Masts are an issue now that the boats are wider and carrying more sail area so we are starting to see them break... Will Sunnucks broke his at Texel in a classic compression failure. He now has a Nacra F20 section.

The Asymmetric foils on Herbie's boat seem to generate a bit too much lift and the transition between modes is too dramatic to be reliable.

So, as Xander said... He sold his M20 and bought a Nacra. The difference is a lot to do with the hull shape and the amount of research and development that was invested in the design, plus the integration and overall feel of the boat. The M20 has moments when you think its unbeatable but as an overall product its hard to beat the F20.