I have used spectra (1/8) for traplines for 3 years and spectra (4mm?) for one shroud and vectran (5mm?) for the other (P19). I like spectra better, the slick finish seems less abrasive and less abraded. I can not detect a difference in stretch. The length seems to vary slightly due to temperature and more from ending of season to beginning of next. I use shroud extender on both sides so I can single hand setup and preload the rig. The slight give is great for setting up a tight rig and yet allow the mast to rotate fully. When you need to make a change in shroud length it is great to be able just respice the thimble (use the continuous type). Even better is to use small dead eyes, I believe their is a guy in CA that is making them, but I gave up trying get his site to work for me.

I have a carbon fiber mast and have had both rudders out of the water with a new crew and pulled it back down more than once. Loosing weight aloft may not make you faster, but it makes the boat handle so much better. You can loose nearly as much weight aloft by going to synthetic rigging as a carbon fiber mast for about 1/30 the cost.

I have lost the mast twice with ss shrouds, so stainless is no guarantee. There are some minor annoyances with synthetics, but overall performance is far superior to ss. I have kept a ss forestay because I have a furler on it and I wanted to be sure where any stretch was coming from.
I have kept my rigging off during storage because now it so easy to do and fear of UV. The UV fear is greatly exaggerated, data from NE ropes indicates that you will almost certainly wear them out before uv has a noticeable effect. Creep is also a greatly exaggerated fear, for use on catamaran rigging. If you look at the actual loads and duration necessary you will have a broken boat before it creeps.