Its to get the exhaurst pipes the same length (helps with performance) and also to get the turbo in the right place, im not sure about the reason for the large collector though.
I still dont understand how he can be using QG18 pistons. If the GA16 block got bored out to 80mm then it would only leave about 1mm between cylinders. My brother had inteneded to do this to his GA and then realised after removing the head that it couldnt be done.
i think the 1mm thickness was only a theory/assumption SJ (rally bob or anobii ?)
i doubt if anyone has actually cut a GA block open to check wall thickness ?
or maybe tinky winky had liners fitted ?