Hi-flowed turbos with OP6 housings are proven +250rwkw turbos.
Off the top of my head I don't know too many GT28 based turbos with a T3 turbine housing unless they are mated to Nissan housings. The HKS GT-RS is one, (the HKS 2835 ProS doesn't count as it is a GT30 based turbo).
The statement about flowing almost 2x as much is completely incorrect.