Yes the 'X' is generally lighter and every salesman will tell you so and that it makes the boat SO much faster - well it does not although it does help! HOWEVER the 15kg claim is a bit of a salesmans line .... "suits you sir" etc. There are many really light
Mk1's and I don't think Rob White was ever in the habit of giving away kg's of resin for nowt !! And there are a few lardy 'X's out there anyway but there will always be some variation.
KEEP THEM DRY - if you have a MK1 in particular then ensure you have extra hatches in it to ventilate it or it will put on the pounds for sure - also consider a wing upgrade - later ones, as put on the X version, are lighter as they are partially carbon.
Also think about a replacement boom as the alloy ones are heavy as well as being mechancally 'suspect' and generally 'c**p' (always were on the superspars equipped boats). I'd guess the spread is no more than 6-7 kg max and unless the concrete mixer was involved or its soaked up a few extra kg's no more. The boat is very flat (low rocker) and as a result it is far less critical anyway.
Mk1 boats often sail with older (nackered) sails as the owners are often on a tighter budget -
Good sails are MUCH MUCH more important to speed that a few kg's of hull weight. (And X rudders are much heavier than Mk1 you will be glad to hear !)

Mike Lyons