[Brion Toss]

Hi there,
You are talking about a "Whoopee sling", or possibly its cousin, the "Yippee". Both are adjustable, and both can be made in HM rope. The former has a normal eye at one end, an adjustable at the other. The latter is a grommet. We make Yippee's out of Spectra for mast lifting gear, and for temporary stays. Yippee's, I believe, are stronger per diameter, and give the greatest verstatility. Whoopee's are tidier and handier for smaller-spectrum variations in length.
The loss of strength isn't a great as one might expect; in a normal splice, the entry point for the tuck deforms the rope just as much as the exit on an adjustable splice. The difference is that the rope doesn't try to regain its original diameter immediately on the standing part side of the throat, and that it shares the load, on the eye side, between two parts.
One important detail when using a Yippee is to position the load on the doubled thickness, right by where the adjusting tail emerges. That way it becomes very unlikely that the tail can be accidentally pulled, slackening the splice.
We bury at least 72 diameters of the rope, usually quite a bit more, as a paranoia-reducing move.
Fair leads,
Brion Toss