You don't have to put a cat curve in the ridge seam to get a tight set-up. Curves on the outside hems will do the same thing much easier. A one-inch offset for each foot of ridgeline will do the job. 1/2" grossgrain on the hems will prevent stretching of the hem on the bias, giving an even tighter set-up.
I have a hypothesis going that getting a tarp good and taught is made more difficult because the hems don't stretch as much as the rest of the fabric. My proposed solution is to use a small zig-zag stitch on the hems and ridge-line, to allow them to stretch more (polyester thread doesn't stretch nearly as much as nylon fabric). I started a thread discussing this idea on bpl titled "Tarp Taughtness" if I recall correctly.
I have yet to test it yet, though... If it does truly work, it would be a lot easier to do than cat cuts.