How much is the decrease?

It's hard to get solid numbers. Roomsharing (which is recommended) decreases SIDS rates by half, which will be the majority of the benefit of a transition from own-room sleeping to cosleeping. It also seems like the overwhelming majority of smothering deaths deal involve other known risk factors, like smoking or drug use by the mother. It's also frequently recommended against the infant sleeping with the father or siblings (by both sides). Epidemiological studies have the issue that cosleeping is officially discouraged.

If you're adding in psychological factors, though, there's some suggesting that cosleeping is good for the infant / their later development.

As may be unsurprising to the cynic, much research on infant sleep is funded by crib manufacturers. My read of the issue is that cosleeping was recommended against because of the known danger of smothering and the social benefit of parental independence from the infant, and that more information is slowly coming to light that the infant is better off cosleeping with the mother, except when other risks are present.