Well, let's compare 3/16 Sta Set (which is not a pleasant line to use) to 3/16 Samson Ultra Light (simply because the specs were easy to find). Both are about the same diameter. Both have about the same tensile strength (1400 lbs vs. 1200 lbs). Sta Set is double braid polyester. Ultra Light has a HMPE core with a polypropolene cover.
Just to make the comparison easy, say you pull on your downhaul line with about 130 lbs force - that's probably about twice as hard as I ever pull mine. At 10% load, Sta Set has 1% elongation. Ultra Light has 0.5% elongation. If that difference (one half of one percent) additional stretch is noticeable and bothers you, then by all means, go for the high modulus line. I myself can't tell the difference by feel. I simply pull until the sail looks good.
Of course, I'd still choose Ultra Light over Sta Set -- because it is easier to grip. For a continuous loop, I'd choose something easy to grip and easy to splice, such as Marlowe Excel Control (which is specifically designed for that purpose).
Compare the Marlow Excel control. I'm telling you, maybe it wears in and settles down but it's almost like a bunji off the roll. Tie a 15 foot piece to a fixture (trailer hitch) and pull on it...it's pretty surprising. Also worth considering, the Samson ultralight is a blended core - not a true high modulus line.
When we're hitting the limits of the downhaul, you start to feel a wall of resistance. That wall is much easier to feel with a line with less stretch. This is a bit of an over-simplification, but if you have a 16:1 purchase downhaul system with 18 inches between the blocks, a 0.5% elongation at 10% load translates into that line stretching about 1.4 inches between the cleats. the 1%, 2.8 inches of stretch. That said, I've found some conflicting information that even amsteel blue has elongation figures near that...and I'm not certain that elongation is the right type of measurement to evaluate the "springiness" of a line. We're also getting into the distinct territory of opinion, but I can definitely feel the difference and the higher modulus lines on the high purchase or long length (spin sheet) lines. Whether or not it makes a performance difference is certainly debatable.
But - I do recommend securing a long length of different lines and comparing how they stretch over a distance by leaning away from them and hanging on. It's pretty dramatic.
