Scooby, you proposed:

“Class X is sailed by many very good sailors in active fleets around the world and so is sailed very well; however they do very little handicap racing, those that do handicap race are those at the tail of the fleet - The portsmouth rating reflects this because it scores where the boats finish, and this MUST be a function of the skills of the crew(s).

Consider Class Y that also has a very good fleet of sailors, but they also do many many handicap events and so the Portsmouth for class Y is a "hard" handicap as the boat and sailors have performed well at events.

Consider class Z. Not very active, and those that do sail sail Portsmouth events and finish at the back of the fleet; over time the handicap of class Z gets more favourable; then Hotshot A comes along, sails Class Z well and cleans up.“

First, Portsmouth compares the speed of all types of boats, not just those that race Portsmouth. Your description of Class X and Class Y still result in the same Portsmouth number because all data from all boats is submitted. The only exception is that the data comes from USSA sailing clubs and regattas, not from worldwide. Portsmouth is a reflection of what is happening in the fleets using Portsmouth, i.e., it services its users.
Class Z is truly being reflected by Portsmouth numbers, and I believe that is one of the things that make Portsmouth better. If a “dead boat society” ends up with a more attractive number over time as a result of the data, that makes it an attractive boat for those new to racing or cat sailing. If they like winning, they will recognize the best way to get better is to race one-design in large fleets, which many consider the best way to “improve the nut on the end of the tiller”. If a great sailor decides to win the Portsmouth game by using a boat with a great number, then they only get away with that for a limited time, as the number will continue to reflect the data. To win by getting the best Portsmouth number boat is a hollow victory and temporary. Anyway, the great sailors seem to gravitate to the best competition, which is what seems to be happening in the US F18 fleet.

Wouter and Scooby, et al: I do not think any handicap system is flawless, however, I believe that the US catamaran racing fleets are best served by using Portsmouth in its entirety, including wind adjustments and the other HC adjustments for changes to the one-design boat. Possibly, Texel, ISAF, SCHRS, et al, do best at serving their catamaran fleets. I am not the right person to judge what is best for you, and visa versa.