Erin and Hig wore a prototype 2-piece Alpacka semi-dry suit. In their blog they said it was a stretch WPB fabric. The only stretch fabric with adequate hydrostatic head was Toray's Entrant Dermizax three layer material. Assuming this was the material used, Dermizax tests 8,000-10,000g/m2/24 using the JIS L1099 B-1 test. By contrast, other breathability results using the same tests are: eVENT, laminated to nylon, yields 27,826g/m2/24 hours; Gore-Tex XCR yields 21,194g/m2/24 hours; and Marmot PreCip yields about the same at 10,000g/m2/24 hours. The weight was never published but the gaskets would likely necessitate a total weight no less than 2 lbs.
My size L 2-piece Mont-Bell/Kokatat semi-dry combo weighs 2.1 lbs. This combo allows a small amount of water in (1-2 cups) during a long white water swim but, not enough to experience cold water flushing.
My size L Kokatat GFER dry suit weighs 3 pounds and is absolutely dry inside after a long white water swim.