Wouldn't you want to advance the exhaust cam so when it opens the air-fuel mixture is still burning? If you retard the exhaust cam it will open later when the burn is further progressed and more of the energy is already spent, leaving less for the turbo. Retarding the timing is the same thing, the burn starts later allowing more energy to be fed into the exhaust and less contained in the combustion chamber.
Ask me how I know... I had a Ford 302 once and the vacuum advance fell apart on the distributor when I was on the highway. By the time I was able to pull off without getting ran over I was greeted with glowing exhaust manifolds under the hood.
