The 2860R has a separator cast into the exhaust housing between the wastegate and the main turbine exit - which a std GTR turbo does not... and I'm guessing that the GT-SS is the same? I could be wrong...
The trust front pipes have this separator as part of the dumps - so because you have two wastegate separator plates, you need to grind off one, and it's easier to do the dump pipe than die grind the exhaust housing