Re: Topping lift loads?
On my former 25' boat, I only used the topping lift to briefly keep the boom from falling into the cockpit while raising and lowering the sail, and when reefing. Except for those brief moments, I had the pig tail hooked up. I unhooked the pig tail just prior to raising the sail and immediately after lowering it. My crew knew they couldn't lean on the boom during those few seconds, and steadied themselves elsewhere, such as at the mast or a grab rail. Because it was so briefly used, I used the lightest nylon line possible. I think it was 1/8 inch nylon. It was enough to do the job required of it, and, if I ever forgot to ease the topping lift after the sail was up, it stretched so easily that it wasn't a problem. If my boat didn't have a pig tail or boom crutch or similar device, then I'd want a light, but strong, non-stretching line.
I'm not an advocate of the "oversized line" theory. I generally prefer to use the size of line that is just right for the task, and no more. Oversized lines often don't run freely through the blocks, and they are often too thick or too heavy to work well. I'd rather have it work properly, and, if I wear it out, then replace it, and use the old line for lashing things or other uses.