Faster downwind in the really light stuff without the spi? Either your spi is too heavy, or you are doing the monohull "barn door" thingy <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
Seriously, if you can't get the spi to fill, you are usually better off without. But racing in those conditions is not much fun and should be called off in my opinion.