I've seen quite a few stainless manifolds for stock turbos(k04),these would require good bracing to avoid cracking with the weight of a heavy gt28rs.I might have seen a cast one in the states but i can't recollect where.
So thats a new manifold,new turbo/kit,new downipe and flange,new oil and water lines,de-cat or sports cat.You would need a new intake hose as the inlet on a gt28rs is 3",adapted outlet/ic hose .Then you would need bigger injectors and have it would all mapped.It won't be that cheap.
quote from site "This turbo happily makes a reliable 275 crank HP on pump gas and close to 300 crank HP on high octane
"-k04 performance.
I've read the other bit about how the stock manifold flows better than the atp manifold-hmmmm
I predict big lag,time will tell.