Evan, few things are as simple as they seem.
To first address your weight question, I realize that the header on the Aircore 1 page says 1.1oz. However, the chart below says it weighs something like 0.004 oz/ft. At fifty feet, that's 0.2 oz, less than your z cord. The 0.2 oz seems realistic to me. I have some 100% Spectra cord at 500# test that weighs roughly 0.5oz for 50 feet.
As Roger and Greg have mentioned, knots are not fun w/Spectra cord. Best way is to tie permanent loops in the ends. If you have to tie off something like a bear bag, it's best to leave big loops in your line to give you something to grab onto. Actually, in bear-bagging w/pure Spectra line I've found it almost essential to tie in hand loops; the line is too thin and slick to hold onto well. These concerns are why you'll find some heavier Spectra-cored, sheathed cordage such as the Kelty Triptease.
Another thing to consider in selecting the cord you carry is its ultimate field strength. In other words, although a cord might be rated to take a static load at 100 pounds, it won't take that weight at a shock load. Real-world example: 500# test Spectra won't hold my 70 pound dog when she sees a squirrel. The cord is static, it has no stretch. Dynamic cord, such as paracord, she doesn't usually break because it does have some give (stretch, shock absorption).
For tarps that kind of loading probably isn't a concern. But if you might get hit by some high winds, I wouldn't count on line taking the full brunt of its rating.
