Do you sail Nacra 5.2? I doubt it would be too different than the 500, other than the ability to play with the daggerboards against the eventual lack of weight (I have no idea how significant that is, I only sailed the 5.0 so far).
I weight about 185 lbs and I've sailed solo in pretty choppy conditions on the 5.0, with no problems. I don't think you should be too concerned about not having enough weight unless you will be mostly sailing in strong winds, I read here that the 5.0 is rather underpowered and that the 500 is very similar to the 5.0 in general. Not having much experience with other cats, I tend to agree, it's not like it will fly a hull for nothting. Personally I would hate to go to a smaller boat, I sailed H14s a couple times and didn´t find it really exciting. If crew weight (and budget) was not a concern, I would certainly be tempted to go to a bigger one, just get some beachwheels to take it off the water durning lunch, the rest shoulnd't be too different I think.