have a look at this Racer Brown article on valve timing. although it is directly related to L series Datsun motors, there is good discussion of all valve events in the 4 stroke cycle which is applicable to all 4 stroke engines and some mention of valve timing for turbo engines in there somewhere, but I can't remember exactly what they said. it's an excellent read anyway...

I forgot to say, the A14 we turboed had a 25/65 cam in it. not sure if that was good or bad, but it was in there before we ever thought about whacking the turbo on.