Lol open a can of worms why not! Many many (and generally poorer tuners), will gas flow heads for peak power, high valve lifts e.t.c, this can lead a piss poor engine.
From memory expect around 5% - 10% (could be wrong!) gains in flow accross pretty much the entire rev range, which will be a directly proportional increase in torque and power. It doesnt sound a lot, but it makes all the difference.
Craig will make the head bespoke to you, you tell him if you want a gas flowed head for rally race engine, or a head for a road car and he will do as per your needs. Most cheaper engine tuners will be scratching their head by now going ... "so you want a stage 1 head yeah?"