Since I've got a PMAG for every other gun now...
I got a PMAG 17 GL9 for my Glock.
They are measurable lighter than the factory magazine. 9.4 oz. loaded for Magpul and 10.5 oz. loaded for Glock.
That should be worth a 0.1 lb. reduction per magazine in GURPS, but the 10.5 oz. is shown as 0.6 lb. (0.65625 lb. to be exact) in High-Tech when it should be 0.7 lb. The Magpul mag is 0.5875 lb., so closer to the 0.6 lb. from the table.
Reality testing can be disappointing sometimes.