cFinch used a .86 GT2871 on his 400whp Sr20 I think, try and dig up that thread. Everyone I know that has hit the 400 number has been using the .86 turbo, because the .64 levels off as you get close to redline. To add to that, every SINGLE person I know that has hit 400whp with a 2871 ends up getting a bigger turbo, like the GT3071 or T3/T4. It just doesnt pay off to run those types of boost numbers with that small a turbo.
Like USDM silvia said with the GT-RS (which is almost identical to the 2871 .64 BTW), those turbos are about hitting 350whp or so safely and with great response ~ if you want great response and 400whp, build the bottom end and put a lil n20 on it
