Thanks for these comments, especially Wouter's which were very interesting indeed.

First, I'd like to point out why I care that much about the weight of the boat. Although I sail mainly with two (my girlfried, one of my sons, a friend), I'd like to have the opportunity to sail alone. One subject is of course the righting the boat after a cartwheel (<- correct word?) - which is with my Nacra 5.0 so easy that I don't regard this with a boat of similar or less weight as a problem. But I have huge problems to get it out of the water. At my "home-beach" I have a steep way out of the water, and it for me, being not too strong almost impossible to get the boat out back of safe land.
Of course, if the boat gets faster due to less weight I don't mind ...

Coming back to the boat in question, the Nacra F17. Here, in Germany, it is indeed available in two versions, with 13,8m2 and 15,5m2 (F17XXL) main sail. However, my dealer confirmed that with next year's model, the version with the larger mainsail will be available only. The current weight is given with with 140 kg (without jib and spinnacker), He guesses the new weight with about 125kg, so you have a complete boat with jib, spinnacker, etc. at around 135kg.
Do you have any information on the weight of a Nacra 5.0? Since this is what it boils down: the reduction in weight between the two boats.

However, it is very interesting what you say about the Formula 16 class. Since you are obviously member of the F16 group: do you have any contact details of Holger Siebke? Kiel is a 20 minutes drive from my place, I am there every second day.

I am pretty new in cat-sailing, and had no intention to look for a new boat. Just by accident I had the opportunity to sail a Nacra F17XXL (2004 model) in my usual conditions and I was surprised at the difference as such - and that I am able noticing it with my pretty little experience. At that point I decided to give the FX One a try too. Again a considerable difference, much better than my Nacra 5.0 - but not as good as the F17. The F17 gives me the impression being a double-hander with single-hander option, the FX One a single-hander with double-hander option. And since I do 95% of my sailing with (a pretty light-weight) crew, I guessed the F17 being the right boat for me.

I must confess that I never had the opportunity to sail a Formula 16 boat (except a Hobie 16 which is certainly not a subject here). It is just an assumption but I'd guess that its ability to go through rough waves at strong winds (we sometimes go out at >5bft with >1,50m waves) is limited compared to a larger boat. Also the mechanical rigidity might be compromised if you have just over 100kgs of weight compared to approx. 140 kgs of a F17 or even 180kg of a Formula 18.
On the other hand I must confess that as I contacted my local Narca-dealer (Sven Lindstädt) he recommeded for my purposes, apart from the F17, the Taipan 4.9 which is a whopping €2000,- cheaper than the NARCA. Is this the boat you're owning?

However, it boils down to a test-sail at my local conditions and if you have any contact-details of Formula 16 owners not too distant from my place, I'd appreciate if you could forward them to me.

Thanks!