Is it then wise to choice a stationary class a your viewpoint
No. I don't think a rating system such as this needs a focal point at all once the formula has been stated. All boats just need measuring and weighing and you plug this into a spread sheet and bingo you have a rating.
Your example of F18 reducing weight or increasing sail is exactly what I think any rating system should not be "sold" as tied to a single boat as the focal point. I think it would be good PR to say that the rule / formula was worked out in this way using the F18 performance as at 2004 as the point of referance, but from that point on all ratings are free-floating. Eg, someone puts a Masthead kite on an F18, you can just adjust the rating as such. I put a bigger kite on my 17, my rating changes too.
The F18 becomes 15Kg lighter - I believe entirely possible as the new Nacra F17 is 13.4KG lighter (I have been told) than the Current Inter 17 (Euro version).