Duomid guylines

That seems like it could be a good idea, but I wouldn't want to use it for the primary lines. I'd be worried that the elasticity could knock down or destroy it if the wind hits it just so. I think I'd add a short length of shockcord to each loop and attach the other end to the existing line with a taut line hitch.

I am currently using Kelty TripTease line for my guylines and the included linelock tensioners. I would keep the TripTease line but maybe get rid of the linelocks and replace them with something like this:
http://www.hammockforums.net/forum/showthread.php?t=3731
Just looking for feedback from people that may have done this and how it worked out for them.

On our previous shelter ( MSR twinsisters ) i added linelocs .
But my new shelter ( paratipi ) has up to 14 stake loops
so instead i used 3" pieces of shock cord.
I made some loop with each and depending to condition / soil i use this loop single or double between the stake and the paratipi.
It helps me having some freedom to stake the shelter and later it helps by maintening some tension

I find I prefer a 3/8" or thicker shock cord. Thinner tends to give too much in winds, causing bouncy, flapping walls.

A 6" length has worked very well for my shelters, but it depends on how much stretch you experience.

I'd suspect that if you want a longer length, you may want thicker shock cord and this would add weight.

I've been using shock cord with my guy lines for the last few trips, and I've ditched them for the next one. I don't find them necessary and they just add weight.

I run my guy lines by doing a quick release not around an anchor, looped through the grommet/tab then a rolling hitch knot back onto the line. I can adjust the tension from under the tarp, and if the line around the anchor slips a bit I can slide the rolling hitch all they way down instead of being stopped at the shock cord.