Well for the turbo you are looking at more cylinder pressure and more heat because of a harder combustion. They all need a flat and clean surface to have a long lasting relationship with the head gasket, whether it is turbo or not. And like some have said in here, ARP studs or bolts that allow much higher torqueing which keeps the head gasket stable and as little movement as possible between the two.The block decking is mainly to get two very clean and very flat surfaces to mate right. Any variance could cause a leak, which the turbo will find quicker usually. You can deck a block to raise compression too by having the pistons come up a little higher, or you can mill the head for the same purpose. Just make sure you get your cam timing right again or you will lose that power you were trying to get.
Russ
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