I am not sure what turbo you are talking about. Something custom or a off the shelf brand name? But here is my guess.
There have been many improvements made in turbos in just the last 2 years. I would look into a Borg Warner EFR sized to produce about 400-500 hp.
My reason, the newer design will spool better, ETT (extended tip tech.) were a vast improvement, so EFR incorporated ETT and Ceramic/Ti so they should be lighter, which means quicker spool and more air.
Or you could go with a small set of twins. But not knowing what turbo (Borg Warner, Holset, KKK, or what) it is difficult to make more than a general sugestion.
And you may want to add a 4K GSK.