I'm running 16:1. My boat came with a 8:1 originally, lead to the sidestays. I doubled the purchase by adding a two dyneema lines from the boom attachment up to the tack and down to the blocks originally attached to the main.

It works, but space is tight: the length of the line need to be calibrated carefully if you want enough travel down, and the blocks occasionally catch up on the boom. Sorry, no pics...