16:1 downhaul purchase is beyond overkill for the stock 18 mainsail. You'll have ton's of extra line strewn about the trampoline. However, I don't think you'd pull the tack grommet out of the sail, you'll just bottom out the system on the gooseneck fitting.

We run a 6:1 downhaul with good harken blocks and dual exit blocks and my crew can work the downhaul from the trap with no problem.

sm